Lew Hodgett wrote:

If I were faced with that problem, I'd build the bench with all legs the
same length/

Then cut 4 extra pieces of leg material, say 12"-18" long.

Put the bench in place and then using a small hydraulic jack and some
shim packs made from say 1/4" plywood, (The shim pack can be say 2"x6"),
level the bench.

When you have the bench totally level. then clamp a 12"-18" piece to
each leg so that it touches the floor and clamp in place with some
C-Clamps.

Wait about 2 weeks, recheck bench for level. If level, bolt leg and
piece together with some 3/8 bolts and large fender washers, then remove
clamp.

SFWIW, ever wonder how they level out a 20 ton boat when they put it in
a cradle or a house when they move it?

Same way.

What I describe above comes under the heading of BFU (Butt F**K Ugly)

Use only the shim packs.

If you make them 3x6 the you can turn successive layer 90 degrees.

Another approach is to drill a hole in the bottom of the leg, then
install a 1/2-13 S/S Tee nut, then screw in a 1/2-13 x 3"-4" S/S
carriage bolt with a lock nut.

If you are on a concrete floor this works, a wood floor, stay with the
shim packs.

Lew