The platen should be 2-3" away from the garment. This is a rough rule of thumb, and can be changed. To get a faster flash time, for example, you can move the flash dryer lower. To reduce the heat on the garment and prevent over-flashing, or to prevent over-heating the platen, you can lift the flash. It is pretty common to see garments requiring less heat to flash when your platen starts to warm up as you continue through production, so keep that in mind.
You may want to consider a temperature control for the flash heater if you find yourself needing to adjust the height of your flash pretty often.