You can get very accurate with a pair of parallel timbers.
I shim this lab countertop to a good tolerance for doing long work with some adjustable blocks.
Adjustable as in those offcut blocks are not totally squared and can be messed around for that press fit.
Regardless if you're planing or shimming to get the bench flat, using a straight edge,
having a pair of beams means you can double the error of the workbench and arrive to a point where you can trust the bench for laminating.
Make sure the timber beams are as long as the bench, otherwise it's very likely that trusting it for longer stock will result in the ends being wastefully nipped off.
Much less likely to be so wasteful if it were the other way round, as would it be more apparent, because the plane won't cut as readily into a hollow.
Make them parallel so you can pair them to make sure there is no spoon factor happening.
To me, that means that it is the slightest of bit of convexity by an unmeasurable amount
so the work sits on the ends, but the gap in the center of long work is invisible, or at least when grasped by light hand pressure.
The only problem is that those timbers always seem to get used up.
Nice to have a lightweight beam of some description none the less.
I have some sort of aluminum beam and it's pretty handy, haven't a clue what it was used for beforehand, sorry.
Plasterers tool perhaps?
