First I would look at the size of pond that 90 acres of watershed can maintain in your area. I'm thinking in the neighborhood of a 9 acre BOW.

IF that is close to correct, then the springs are just an added bonus for a larger 9 acre lake. A 4 acre pond may get overrun with excess watershed. My little pond has this problem. (1/4 acre with 20 plus acres of watershed)

You can research the evaporation loses in your area, calculate the loss in water from your anticipated pond size (in gallons), and then compare it to your average spring outflows.

This is a very interesting calculation and I hope to read more as it develops. Sorry I can not just whip this out for you, but there is quite a bit to it and a lot of it has to do with your area and pond environment.

Fish on!,
Noel