Well, when I dry-fit it, there still a gap I can't get rid of; the gap is
uniform all across, so my cuts are good. I marked up a biscuit with a
central line, and the central line falls exactly on the edge of the slot.
I have thinking that maybe the clamping on 90degree joints is adequate to
make it work, but the clamping on 45degree joints won't push them
together, and I have to make my slots a hair deeper.

I went though 100 biscuits and pulled out the narrowest ones. They varied
from 1.917 to 1.945, with most between 1.924 to 1.931.
Everything fits fine now.
I even upped the count from 2 per joint to 4.

One of these days I will increase the cutter depth a couple thousanths, but
it hasn't been a problem except on the angled cuts.