Planing is what ultimately limits the height for a general-purpose bench. It needs to be low enough to plane comfortably, or your arms wear out quickly. However, too low and you get a sore back from stooping over too much. And a good planing height is a little low for everything else.
Because everybody's built a little differently, there's no standard measured height. Instead, it's better measured in relation to your own body. Chris Schwarz likes his bench to come up to the point where his pinky joins his hand. Roy Underhill likes it to come up just under his knuckles when he makes fists. And Mike Dunbar likes it to come up under his palm when he tips his hand up flat.
At some point you just have to pick a height and go for it. You can hedge your bets by erring on one side or the other. Keep it on the high side with the expectation that you might need to trim a bit off the ends of the legs to lower it. Keep it on the low side with the expectation that you might need to add some kind of feet or risers to raise it. I chose the latter, using the pinky measure.
After using the bench for about 6 months now, I decided it was just a bit too low. After more obsessing, I figured raising it by two 3/4" thicknesses would be better. But I wasn't totally sure. So I came up with these slip-on feet that can have loose shims added or removed for fine tuning.
I opted for the simplest possible construction here, since there's no need for any structural strength. The feet just need to keep the shims in place under the legs if the bench slides around a bit. They're simple nailed boxes, about as sophisticated as a kid's birdhouse project.
Ripping a scrap of 3/4" CDX plywood for the bottoms. A good sharp crosscut saw cuts plywood and MDF just fine.
Crosscutting the bottom to final size. Wait, is this crosscut or rip? That's not really even meaningful with plywood.
The bottom pieces slipped under the legs for testing. Time to do some obsessing and see if this is a good height.
After cutting a couple of short sides from a scrap of pine 1x4, marking out for the overlapping long sides.
Crosscutting the sides. I didn't bother shooting the endgrain, no need for that kind of precision here.
Using a bradawl to make pilot holes for the nails. Otherwise this pine would split, nailing so close to the edge.
Driving the nails on a short side.
Nailing a long side.
An alternative for making the pilot holes: Yankee drill. Using the awl can give you sore wrist, forearm, or elbow from the repetitive twisting motion.
My concrete basement floor isn't perfectly flat, making the bench wobble (it couldn't possibly be that my bench is less than perfect). In addition to the second layer of CDX inside this foot, dropping in a layer of fiberboard to shim it up.
Levering up the bench while slipping the feet in place.
Still just a tiny bit of wobble, rectified with a few playing cards slipped into the opposite corner foot. Note that the front feet are open on the front side. On the left front foot, that keeps it from interfering with the leg vise. On the right, that keeps the plane of the bench front flat. If these feet shift around, I can put a screw in through the side.
The feet in place, all shimmed up and rock solid.
Ok, so they're not as graceful as Cinderella's glass slippers. More like big ugly boondockers. But they get the job done. I'll use the bench like this for a while and see how I like it. So far it does feel more comfortable while planing.
The final height? It comes up to the point where my pinky joins my hand. Wait, you ask, wasn't that where it was before I added the feet? Yeah. It turns out you shouldn't slouch when you're measuring for bench height.