I had a set of eight such chairs that all needed tweaking, so I built a
jig with some scrap wood that let me use a router to level the feet. It
was just a square of ply with some holes cut at the feet, and two pieces
of pine to support it just above the feet (they rested on the
cross-braces, chair feet-side-up). The router had a straight bit with a
template follower (just to avoid cutting into the plywood by surprise).

For each chair I flipped it onto the bench, clamped on the jig, and
routed the four feet.