Hi all you math geniuses out in Jennyland

I am using a bunch of charm sq in a quilt top making a lot of hourglass blocks. I am squaring up to 4" which means 3.5" for finished size. I'm going to need a gazillion blocks bc the size is so small but that's not my math question.

I think I want to use HST for a dramatic border but I.don't know how big to make them so they will be in the right proportion to the blocks. Also the top isn't finished yet so do know the final measurements. Should I have that number first and then divide it mathematically by size of HST blocks so the numbers come out even? I'm not sure how to do this. Thanks for pointing me in the right direction.