If the leg levelers will not get the frame close, use something solid like slabs of metal, or hard wood. Hard wood is easiest cause you can size them easily (planers).
Once the frame is level, do what the others suggested...
I used to use a machinist level but it was WAY to sensitive to use. It took too long /shrug I have since been using the raw slate to shoot/roll a ball on. Roll it in all directions and if it roll straight on slate, its gonna roll good on cloth... Even if the best you can do is get a slight roll on one plane, no worries... Once cloth is on, its quite amazing how true they roll...
Be prepaired to compromise no matter how you decide to level it. Most slate has some sort of problem... Cloth will even it all out, most of the time...