I'd keep feeding them until they ovy. Feed them well while they want it.

We have ultrasound. It's the first year I've used it, though, so I'm still learning. It seems that as they start to really pound the food, the follicle growth rate increases. This seems to be in the 15-25mm range. Still haven't had an ovy, but we should be close. The one closest has 26mm follicles. She only has 4, and she's a smaller girl (1100 grams), so I don't know what to guess as far as what size they'll be when she ovulates. Time will tell.