Honest answer: One option is to just cut it roughly to start, then use the washer technique to scribe a line. For a circle (vs. a "straight" line), you'll need to then use a compass to reduce by the width of the washer or use the washer to make a series of marks that represent your cut line.
Which is why I added to my comment before you made your comment that you just have to use the washer to offset the line once you've marked it. I've done this dozens of times and it's easy.
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u/billybl4z3 2d ago
what if I don't have 90 degree corner behind