The Gemara says (mesechet shabbat 31b) that one can survive with a rivi'it of blood. How is it possible when we know one can't even survive with a 40% blood loss?
Rashi indeed seems to indicate like the OP’s assumption:
רביעית דם - חיי אדם תלויים בה
A Revi’is of blood - the life of a person depends on it
The Gemara in Sotah 5a takes this even further:
א"ר אלכסנדרי כל אדם שיש בו גסות הרוח אפילו רוח קימעא עוכרתו שנאמר (ישעיהו נז, כ) והרשעים כים נגרש ומה ים שיש בו כמה רביעיות רוח קימעא עוכרתו אדם שאין בו אלא רביעית אחת עאכ"ו
Says R’ Alexandri: any person who is haughty, even a slight breeze bothers him, as it says (Yeshaya 57:20), “The wicked are like the troubled sea.” If the sea, which contains many revi’os [of water], a slight breeze bothers it, certainly a person, who only has one revi’is!
אדם שאין בו אלא רביעית אחת - שברביעית דם הוא מתקיים שיעור זה הלכה למשה מסיני שרביעית דם מת מטמא באהל מפני שהיא נפש וקרינא ביה על כל נפשות מת לא יבא:
A person, who only has a Revi’is of blood - With a Reviis of blood does a person survive. This amount is a Halacha l’Moshe miSinai, for a Revi’is of blood is enough to give off tum’ah in a tent, because a Revi’is is considered a ‘nefesh’ that we may apply to it ‘to any dead nefeshos he may not approach.’
But do not fear; this Gemara in Sotah is so surprising that Tosfos had to take notice. And they do provide an answer (rough translation on my part):
אדם שאין בו אלא רביעית דם - פר"ח הוא דם הצלול שממנו משתית הלב אין בו אלא משקל כ"ה סלעים שהיא רביעית הלוג:
A person, who only has a Revi’is of blood - Rabbeinu Chananel explains that this is the clear blood which is contained in the heart. It weights only 25 Sela, which is a Revi’is.
So the Gemara isn’t saying that a person can survive on just a Revi’is of blood, but rather than a person’s heart is constantly pumping a Revi’is of clear blood at a time.
Clear blood may refer to the plasma, ie the liquid where red globules are swimming. It represents about 50% of blood.
How does this measure up? According to Halachipedia, a Revi’is is between 3 and 5.3 ounces. This site estimates a heart to be around 280 mL, or 9.77 US oz. So when take half of this for the plasma part, we get close to a Revi'is. Perhaps the difference can be chalked up to estimating on both ends. Perhaps it’s that Tosfos wasn’t referring to the entire heart, but just one or two quadrants of it. Perhaps hearts were physically a different size back then (not likely, at least not to a significant enough degree). Perhaps Tosfos were referring to a younger age than adult - at the opposite extreme, the above link claims that an infant has 8.5 ounces in its entire body.
Assuming your affirmation that "<60% : lethal" (--> sources?)
In math this is called a necessary but not sufficient condition.
Our Sages knew that, the smallest vital amount of blood is a revi'it. In other words, a man without that amount can't live. (This don't imply that "a man with that amount must live".) A counterexample would only consist of finding somebody that survived without.
Maybe you would ask: "But why give this necessary condition, if to fulfill it, one must go below 60%?". And an answer could be: for a baby who has a total volume of 1.5 revi'it, where condition ">1revi'it" not included in ">60%".
See also Nazir 50a
לא נצרכה אלא לנפל וכו'
or Yoma 79a
וקים להו לרבנן דבהכי מייתבא דעתיה בציר מהכי לא מייתבא דעתיה