if the drop-rate doesn't change after a couple of failures, or with level (and as far as I know, that's not the case)... that's all that matters. What else do you want to include, the price of crude oil or the weatherforecast?
The list states the chance you still havent found granite after x-searches, when you start with 0 searches. Workd fine, it's really not a complicated formula at all.
Take the coin. Now how big is the chance you get side B 3x in a row? Yup .50^3 = 12.5%
throw it 100 times. That gives you (statistically) 50 * B as first throw
From those 50 throws half of them give a B the 2nd time -> 25
from those 25 throws half of them gives B agin -> 12.5
Ooh see, 0.50^3 =12.5%