Using "Motocalc"
If you choose your NiMH batteries by physical size, and not by mAh capacity, heating is almost never a problem.
Say you're flying a Zagi 400. The stock battery pack is an 8-cell "2/3A" NiCd pack with 500mAh capacity. To properly replace it with a NiMH battery, you need to find 2/3A NiMH cells, not a NiMH battery with 500mAh of capacity.