well if your able to use a air cool reflective hood then you will be able to get your MH pretty close.....i can actually touch the glass surrounding my bulb. and its just warm to the touch...i have great ventilation......but if you dont have good ventilation, your gonna have to keep the bulb a good distance from the sprouts....but since mine is well vented, i keep mine about 2 feet from them.....tho this is my first indoor grow, they seem to be loving it........
but yes, as soon as they break soil, put the light on them