RE: How to get a perfectly flat (=plan, leveled) workbench?
I usually lay ceiling tiles on my build table to pin to. If your table does have a slight high or low spot, I have in the past used thin carboard to shim the lows out, then once it checks out flat with a long straight edge I will tape it down... So if your hard surface of the table isn't perfect it can be tweaked to be useable.