Hi, i have build my own sheet aswell, so i know this problem. The best way is of cause to get one of the database dump files that are released ones in a wail.
But i run in to this problem when there are just a few new BPO released, then of cause i don't want to redo my entire database.
So here is what i to.
I have perfect production skills, so the only waist i have is the ME. What i do to find the perfect amount is like you do here. but instead of using .90 you should be using 0.90909090909090
and whats even more important is that you round the number before you do any other calculation with the material amount. This will make you hit the correct number.