I had the same problem when I first started feeding my BG... I though they would never figure it out but if you consistently feed them at the same times they will figure it out and be waiting.
I feed at 7 am, 1 pm and 7 pm, the BG are always prompt and they feed as aggressively at 1pm as they do early or late.
They learn the times well and if you throw feed between feedings very few will show up so stay consistent.
I don't really have to worry about too much feed, the turtles will clean up the leftovers, I think I've pulled all the turtles from every pond around me... sigh...
