Thanks for your comments
"Do I have your method properly laid out? I understand using the lines at the "bow" to measure your T/A distance. That is pretty clever and represents an interesting shortcut.
Yes that's the idea
There is a dilemma for me at step 4 however. I cannot understand how you keep the angular orientation of the bow of the protractor constant while "shimmying" it about. A small angular error can yield a large error in fix, if the angle between intercepts is small.
One way to maintain angle would be to set parallel rules across the bow of the protractor. Then pin one side of the parallel rules, moving the other side, such that the intercept distance is indicated by your "bow lines". It uses every part of what you have suggested, but maintains the angle!
If you wish, why not?, and the parallel rules could also be set along the side of the rectngular Breton type protractor for the same purpose
Still no extraneous marks. Still no divider pin pricks. "
The shimmying about is only what you always do with a Breton type plotter, because it has for the purpose the multi rectangular alignment grid within its compass rose, used for aligning the plotter in all forms of navigational bearing measurement and plotting. No difference in plotting accuracy there.
I bought and marked up a cheap and cheerful Weems and Plath Compact Parallel (roller type) Plotter 060 and added the lines added to its 'bow'. All good, but the +/- 90 degrees is done in the brain, and the brain also needs to sort out the Towards/Away matter.
The Douglas type Protractor has been tried, but by its nature it has bearing markings on its edge which confuse matters, and in needs a copious supply of meridians which you don't get on a plotting sheet.
What is going on with the scheme for the Breton type plotter is that the markings work well for Towards intercepts, and the procedure for 'turning the plotter upside down' converts an 'Away' into a 'Towards' on the reciprocal bearing without any brain work.