1 : 1 for the first 4k is correct, after that TG doesn't tell us anything and I never collected enough data points to inspect the formula further.
A "formula" similar to yours had kinda close results to 1 or 2 random points I tested it with, but that doesn't mean anything at all. Unfortunately, the amount is calculated serverside and the formula isn't anywhere in the client side source code. :c
If someone did the work and collect a big amount of data points and analyzed it, I'd appreciate a formula aswell. xD