>>mrheos+(OP)
You simply do not. You do the math yourself to calculate 2(n) for n in [1, 2, 3, 4] and get [2, 5, 6, 8]. You plug it into your (75% accurate) unreliable calculator and get [3, 4, 6, 8]. You now know that you only need to recheck the first two (50%) of the entries.