The Netziv, (in Bircas HaNetziv on the Mechilta, Beshalach 6) says:
"Even though the word "etzba" was said regarding the plague of lice alone, it was meant to refer to the whole process of going out of Egypt (all of the plagues as a group whatever they are). It was a general statement by the sorcerers of Egypt that "all of these events are the work of Hashem and not magic."
So we can say that the sorcerers use of "finger" had nothing to do with the third, or the passing of three, plagues. It was simply that the third plague was when they noticed Hashem.
Therefore, it doesn't matter if there were 3, 10, or 27 plagues in the group; it would all be called "finger". It also doesn't matter if the Egyptian sorcerers knew how many plagues there would be. The statement simply means that the sorcerers knew they were dealing with Hashem, and not magic. It was not a statement meant to convey Egyptian mathematical calculations. We are not dealing with some Egyptian priestly scroll which holds that 3 acts of intervention on behalf of a god = a "finger's" worth; and two acts of a different god = a "toe" etc. That is not what is happening here.
(Also, we need not be bothered that the single plague of cattle disease is called "hand" and not "finger". The Zohar on the parshah, discusses this question, and explains that since there were 5 different kinds of animals stricken, the word "hand" was chosen as the proper metaphor within that plague. This does not mean that the other plagues count as a finger or hand each.)
Also, we do not need to be bothered by the Haggadah's comparison of what an Egyptian said vs. what the Jews saw, in order to make the 1:5 ratio calculation. This is because a Medrash will take notice of an extra word in a verse to establish a point of revealed wisdom. In this case, the exact quote of the word "finger" was not needed for the Torah to express that the Egyptians finally admitted that Hashem was involved. That could have been conveyed by quoting the sorcerers simply: "It is G-d!". The word "finger" is extra in as much as the Torah does not always quote everything everyone said (even if that's what they said).
So, R' Yossi is pointing out that the Torah itself must be endorsing the word "finger" as a proper and necessary descriptive of the situation. It is to teach us that the entire episode of plagues was merely a "finger's" worth of Divine intervention. Now, using hindsight, we see that there were ten such plagues, and at the sea Hashem uses the word "hand", so we can now calculate the revelation of Hashem's intervention on our behalf was much greater than what He showed us during the plagues in Egypt proper.
However, we can still be bothered that the Tanach, in a few places, refers to the entire "going out of Egypt" as "Hashem's hand". For instance Devarim 26:8 "And the Lord brought us out from Egypt with a strong hand and with an outstretched arm, with great awe, and with signs and wonders."
so why make the comparison of "finger" to "hand" by the sea? Why not say that based on Devarim 26:8, the whole batch of "signs and wonders" is called "hand" (not merely "finger") so that the ratio is 1:1 and not 1:5??!
So it seems that the same Netziv (in his previous paragraph right before the one quoted above), answers this too. He says that the key wording by the "hand" at the sea is that "Israel was afraid of Hashem" (this phrase is not used to modify "hand" in other places like Devarim 26:8). What extra level of fear of Hashem was achieved at the sea? He answers that the fact that at the sea, each slave saw their personal master punished and killed in a certain specific way that matched his crimes/level of evil was a totally convincing event, proving that Hashem was indeed intricately involved in everyone's life, and a caring judge of each man's actions in this world. This could not be seen by the plagues in Egypt which were events of general destruction.
So now we can explain R' Yossi to mean that the general proof that Hashem controlled every aspect of nature was seen during the plagues, while the fact that Hashem judged man individually and fairly, measure for measure, was seen at the sea. The former is called "finger" and the latter is called "hand". Hence 1:5.
Still, we can finally ask, if R' Yossi meant 1:5 mathematically or is it just a teaching to contrast intensity? IOW, did R' Yossi have a secret list of the 50 plagues at the sea that was somehow lost and never published in the Mechilta?
IMHO, it seems that an exact list of 50 (although possible) is not R' Yossi's intention. I draw this from some Torah examples. 1. Yosef tells the Egyptians that it is enough if they pay a fifth of their crop to Pharoah to show he owns their lands. 2. The Torah asks that we pay a fifth if we wish to redeem maaser sheni. 3. If someone steals from Hekdesh (me'ilah) he pays an additional fifth etc. The concept of the fifth seems to be used by the Torah to convey that it represents just enough to convince someone, that the matter is important.
So too, the only other time in all of Tanach (IIRC) that the words "finger of Hashem" is used, appears in connection with the writing of the 10 commandments. (see Devarim 9:10 "And the Lord gave me two stone tablets, inscribed by the finger of G-d,...") Why isn't it written by the "hand"? When we say an author wrote something, do we say it was by his "hand" or "finger"? So too we can say here that the Torah may wish to convey that the 10 commandments is enough of a collection of Law to show how Hashem wants you to respect Him and others in general. The rest of the 613 would be the "hand" (the full details of those ethics-interestingly, the Rambam calls his book on all the mitzvos: "Mighty Hand"). This is true, even if 10 x 5 = 50 and not 603! It is a parable. a finger represents a person's willing involvement to a limited extent in a given situation. A hand represents a total manipulation which proves a person's intimate involvement and reveals the complete depth of the involvement.
Maybe the plagues in Egypt could only reveal that Hashem controlled the natural world and there was indeed judgment in general (one fifth). So now, maybe Moshe was just a very good sorcerer who knew Hashem's rules of karma, and invoked them as general calamities(like Billam)? However, once the Jews saw that Hashem was individually judging each Egyptian with exact measure, they feared Hashem realizing He was constantly involved in everyone's life; and they believed in Moshe completely (and they knew he did not use "magic").
"10" in Egypt showed us Hashem is involved. "50" is full (shleimos) involvement. So it would seem R' Yossi uses the key number 50 to represent "fullness".