The Rambam changes his language in two places where he discusses our awareness of G-d's existence.
In the introduction to the 10th chapter of Sanhedrin, where the Rambam lays out his 13 Principles, the Rambam discusses "belief" in Hashem's existence. (Depending which translation you look at, the term "belief" is in the text of the Principle itself, but in Kapach's translation it is not in the description of the Principle, but is in the Rambam's concluding paragraph, in which he says about all 13 "belief.") The Rambam (in Kapach's translation) sums up with:
וכאשר יהיו קיימים לאדם כל היסודות הללו ואמונתו בהם אמתית הרי הוא נכנס בכלל ישראל
"anyone who believes these 13 principles is not an apikorus.
On the other hand, in Yesodei Hatorah, both in the koseres and in the first halacha, the Rambam says that the mitzvah is to know that G-d exists:
יש בכללן עשר מצוות--שש מצוות עשה, וארבע מצוות לא תעשה; וזה הוא פרטן: (א) לידע שיש שם אלוה
The first Mitzvah is to know that G-d exists.
(In the Sefer Hamitzvos many printings have "להאמין" by the Mitzvah, but the mistake in that translation has been pointed out numerous times. See the notes in the Frankel Rambam there for a nice summary.)
R' Yaakov Weinberg explained that the discrepancy is because in the 13 Principles, the Rambam is not listing Mitzvos - he is making statements - if you believe these 13 things, you will not be an apikorus. For not being an apikorus, belief is enough. But the Mitzvah of awareness of G-d is to know that He exists - to take that belief and develop it into knowledge.
As an extra credit point, R' Weinberg explained why both sides of this contrast make sense. The Mitzvah has to be to know, because you cannot have a Mitzvah to believe in G-d - if you believe He exists, then the Mitzvah is unnecessary, and if you don't believe He exists, then you won't be accepting a commandment from someone you don't believe exists. The Principle is to believe, because the nature of the Principles is forming the framework in which you will not be able to justify for yourself to not serve Hashem. For that, belief that He exists is enough.
However, this just means that 100% is not the requirement to leave the category of apikorus. If someone is just playing the odds, but doesn't really believe that He exists, that would not suffice. They would need to have strong enough belief to not be able to convince themselves that He doesn't exist when it becomes convenient to do so. (I can't put a percentage on that #.)