You want the ink to just start gelling, so if you touch it with your finger, it's "dry", which will let top colors stick to the base better. Too much heat and the top colors will start washing off in the laundry. We've never used a device for that, just "touch". Start low, increase the time until it works, print something, take it home and wash it a few times, that should tell you. Obviously, less dwell time under the flash means more shirts at the end of the day.
Steve