Put an indicator in the spindle.
Move it to one corner of the table, zero the indicator. I also mark zero on the table with a sharpe so I don't get lost.
Move the table under the indicator to each corner and take a reading. Again I write it on the table.
As you adjust the feet the table will square up under the indicator, Just like your indicating a fixture in vise jaws, in this case you would use a shim to bring a corner up, or your dead blow to seat it better. Same principle. But now your shims are the feet on the machine.
Go slow, make a move with the feet, then check all four corners again. rewrite your new values on the table. Repeat.
Your total indicator reading should be .0005 or less, when you get there lock the feet down with the jam nuts, and recheck.
Level isn't important, it just dictates which way the coolant is going to flow in the bottom of the bed.
If you don't achieve the .0005 reading you run the risk of twist in the Y axis rails. This will cause premature wear or possible binding, both issue are not good.
This info was passed to me from a very knowledgeable Haas Tech sitting in the parking lot of the Haas factory waiting for parts just like me.
Good luck.