It doesn't matter, as long as it's exactly the same size on each side.
You can really vary how the design turns out by changing the angle.
Something to play with, maybe, on the next one. ;-)
However, you can see the proportions here on the tool itself.
http://oneygirl-myquiltingadventure....ster-tool.html
Looks likes it starts about a 1/5 of the way down the side.
On an 8.5" ruler, I'd call it 1-3/4" .