starter question

I read a lot of information but have not found the answer that I need.

Most of the information on the web, both written and calculators, talking of making a starter from yeast 100 billion for pack, unfortunately in Argentina, we are not able to get large packages, our suppliers bring to Wyest Propagator (about of 25 billion cells of yeast).

The question is how the steps to reach the required amount of for example 515 billion in yeast cells (counter example: Ale / 1075 / 10 gal) with only one pack of Propagator?

There a mathematical mind to let me handle these variables without requiring web calculators?

Sorry for my bad English.

Javier