How to correctly level a raised floor built on an old garage floor?
Get a bunch of 6&quot; x 3/8&quot; galvanized lag screws. Pilot though your joists at intervals with a 1/4&quot; or 5/16&quot; bit, then run screws in to lift and level the joists. Cut of the protruding portion flush with a hack saw.
With that room size, every joist needs at least two support points (plus the outer ends). Don't span more than about 4'.
I would pull a carpenter's line down the center of the room, spaced above the joists with blocks at each end. Match the space all the way across.