I think you are right. I was following the printed directions in the package and the drawing was not clear on that point. Will try again.

I was probably doing OK before using thread/string, I'm telling myself that this tool is more accurate . Kinda interesting that they include info on setting up 2x and 3x installations only, now that 1x are becoming more popular...

EDIT: OK, tried mounting not he outside of the chainring.
(A) The spacers on the cylindrical "clamps" are such that when in place, the bar will not touch (sit flush with) the outside of the chaining, probably by design; the difference is about 3mm so that impacts any calculations.
(B) The clamping pinches the chainring in a groove about 2mm deep at the maximum point and only a few mm across. I spent 20 minutes and could not succeed in getting both on, trying to get the second one on invariably caused the first to pop off the ring, regardless of whether I tried "front" or "rear" first, and I do not have enough hands to hold all the parts in place and tighten simultaneously.
(C) I'll try again later... or just go back to string.