Hi Joe,
I've used the GM to size a knot before, kind of. See headhunter thread here. If you match the strand width size to your strands, then set the strand gap size to three times that, you'll be close if you're tripling the passes, maybe a little short as your bights will only show 1 width on each end.
If you want to try some math, Sidney Wood has a lot of stuff I posted here:
See post# 7 here.
I hope this helps.