Yeah, I live on a hill. I used to worry about the regen cut off. I would be really careful not to start my commute at 100%.
Then one day I was at the top of this tremendous hill. It dropped like 1000 feet over a mile of forward travel. At the top of the hill I was at 68% battery, at the bottom of the hill I was at 68% battery.
I dont think the powercore is a very efficient generator. The best thing to try is to look at the battery level of your board at the top of the hill (start at like 80% or something, just in case one or both of our boards aren't normal), then ride down it like you normally would. Look at how much the battery charge went up.
The board will cut out before it goes over 100%. For my commute the, hill at the beginning doesn't generate 1% battery. By the time I get to the stop sign at the end of my block I'm at 99%. I know I wont go over 100% so I don't even sweat it anymore.
As far as adjusting your speed to reduce the regen, remember that the regen is from the drag on the motor acting as a brake, not the rpms. The more drag, the more regenerated energy. So just ride at your normal speed.