Make sure you blade and your arc point are in a strait line, and measure from there. If you don’t, you’ll end up with an uneven half circle that wont be perpendicular to the other cuts.
Note, this jig wont work with this jigsaw because the blade doesn’t rotate. You need the blade turned at a slight angle for this to work.
I’ve done this before to make a sure and steady cut, but it didn’t work well with my cheap-o jigsaw. If you had a scrolling orbital jigsaw so you could adjust the blade angle to match the arc of the cut it would work fantastic — but if you own a scrolling orbital jigsaw, you probably don’t need my help. I just made this jig from the scraps I cut off of my main board.
