Yeah I may have spoken to soon. There's a minimum bound, but now that I think of it there's generally no reason to believe that any subset of outcomes for any b

Yeah I may have spoken to soon. There's a minimum bound, but now that I think of it there's generally no reason to believe that any subset of outcomes for any bounded number of flips would have a probability of exactly 1/2. In fact this is impossible if p is transcendental. The set of p for which there is a method to do it in finitely many flips is quite interesting though. It is bigger than the irrational numbers (it contains sqrt(1/2)), but smaller than all algebraic numbers. At the very least you'd need 1/2 ∈ Z[p], but that might not be enough.