hi again jonathan
i never use the masks you suggest you might be using,
make sure when it is closed and 1 square is open, that it doesn't fog
the rest of the paper, sometimes that happens ... ( light is sneaky )
another thing you might consider doing
is make a handful of identical prints, and expose them onto a large sheet of paper ( 11x14, 16x20? )
and make a large paper inter-negative. then you can find your exposure time with the paper negative
( it might be room light on, count to 2 or 3 shut off )
soup the big print, cut the little ones out .. and you are all -set.
i tend to shy away from the easels with a mask because the ones i have used didn't keep
the other views safe. but when i made my own out of 8ply matboard it seemed to work ok-sorta.
good luck !