Well, I performed a test (finally) on Ilford PanF+ (@E.I. 50) using Ilfosol-S at 1+14 dilution. I shot a basic scene that was in direct sunlight with quite a bit of contrast.(posted 8345 days ago)What I found is that with 450mL of solution at 1+14 dilution I still obtained strong negatives that are quite easy to scan (I exclusively print digitally). Since the scene was rather bright and the full roll was exposed (therefore, a fair amount of exposed silver, not just five to ten frames), I'm going to go out on a limb here and say that you can get away with as little as 15mL of stock Ilfosol-S per roll (as the volumes were 30mL of Ilfosol-S and 420mL water).
If somebody else would like to refute this please put me in my place. :)