For your design purposes, the fully-charged voltage of a Ni-MH cell is around 1.4-1.5V. However, if you're charging at a high rate, the voltage during charging will be above this. I can't tell you how much higher.
So, you might want to run a little experiment with a NiMH battery and a variable-current power supply. Set the power supply to 1.8A, and chart the voltage on the battery as it charges. If the battery gets hot to the touch, stop charging. You could try the same test with 10A (for a very short test!) to see what voltage you'd need to push current into the battery at that rate. Start your charging tests with the battery discharged (voltage < 1.2 V).
I don't recommend this method of charging, but it's kinda cool to think about. For example, those mini-RC cars apparently charge their NiMH batteries in 45 seconds! That's like an 80C charge rate. I think they are doing something similar to what you're trying to do - they aren't trying to maximize the amount of charge stored, they are simply dumping as much into the battery as they can in a very short window (nobody wants to wait an hour between sessions playing with those little cars, even if it meant that they ran twice as long), and assuming that they won't overcharge the batteries.