How do you compute over/underhang? I'm riding a Mondo 25.5 Oxygen
Protons, which actually have soles that are a couple of mm longer than
my 26 Raichle 224s.

I don't really *compute* it. I just click my boots into the bindings,
and then hold the board perpendicular to the ground, and then try to put
a ruler at 90 deg to the edge.

Same here - not computing at all. I just mount the bidning but the top
plate loose, put the boot in the binding and rotate the top plate until the
boot is flush with the edge. I do this with the board leaned against the
wall at an angle, boot facing inward, so I'm looking at the base.... and
find the lowest angle such that you can't see any boot or binding when
looking at the base. Close enough for gov't work. This is how I choose
the "starting out" stance on a new board... set the back binding up so its
flush and then add another 3 - 5 degrees to the front.

How much does a wide board make
a difference? Like is a 19 cm board much quick edge to edge than a 21
cm board?

I don't think the edge-to-edge issue is that important. Better to
figure out the stance angles you like, and then get a board the right
width for them. Alternatively, if you want to ride powder you have to
figure that into it.

Works best if you have a demo tent at your disposal

Ideally I'd have alpine boards with a few different waist widths for
different types of snow. On hardpack I prefer an 18 cm waist because
edge-to-edge response is noticeably better and more important to
performance. For days when it's snowing in the AM and the groomers have a
layer of snow on them (very common at Mt Hood) I like a wider waist and a
little boot underhang... this way the board clears a better path for the
boots to follow. Same for carving the corn snow.

I find I'm comfortable with anything from 54/48 to 66/63... higher being
better for aggressive carving due to the fact that higher angles tend to
favor heelside which I am weaker at.

Mike T