There are many occasions when it becomes necessary to trim the bottom of a leg to level or remove a wobble in a chair or table. In this case the chair had been used for a long time with lose stretcher and leg joints resulting in uneven wear to the legs, after the chair was reglued it rocked on two legs. If used in this condition it would result in the premature failure of the joints again. This is a quick and reliable way to level legs.
The first step is to determine if 1 or 2 of the legs need to be trimmed, in some cases it is appropriate to graft additional material to the bottom of the legs and then level them.
In order for this method to work you need a flat level surface to work on, either a bench or table saw can be used. I use a small level on the front edge of the chair seat to make the decision of which legs need to be worked. For this chair I determined that I needed to trim one rear leg to make the chair sit squarely and not rock.
The heart of this system is a set of plywood 4 inch squares one with a two inch hole in the center. It’s important that the squares are all the same thickness that’s why I used plywood.
To use put a square under each leg, the leg to be shortened in the square with the hole.
Keeping the three legs not to be shortened in contact with the squares and using the square with the hole and a marking knife, mark the long leg.
A thin sharp marking knife is important to get an accurate marking. If two legs needed to be trimmed I would have shimmed the second leg to split the amount to be trimmed between the two legs, shorten the first leg, then mark and shorten the second leg.
You can trim by either sawing or you can use a plane in a vice level with the bench top. If using a plane chamfer the leg to prevent chipping the edge.