Light Distance

Sup ,Question, how far above my 8 inch tall plants should my 250 watt MH be ?

I have mine about 15" to be safe. Long as your canopy is 75F or so and you have good ventilation you should be good.

Put your hand at the top of your plants if it's hot on your hand it's to hot for your plants. I keep mine just a little above what feels warm(not hot) to the back of my hand. The closer the light the better the plant grows

As close as possible without frying your plant.
Do you have good air circulation?
Having a fan will help take the heat off the plant.