600 watt ballast on 75% or 50% bulb?

I have a digital greenhouse dimmable 600w ballast.
What can I get for MH bulbs if I only want to have it on 50% or 75%?

Also, what size cfm fan is suitable for a cool tube with 600wat and 7x7x8 room?

Thanks a bunch!

U won't need a cool tube with one light if u got a decent size exaughst fan. If ur running at 50% or 75% don't have it too far away cos they will strech loads