Amazon.com: Weems & Plath Marine Navigation Protractor Triangle with Handle: Sports & Outdoors
I just bought this protractor triangle and it seems to be very handy.
I've tried all the other products, parallel rules etc and they all work fine on a chart table.
The protractor triangle seems like it will work on a knee.
Just keep the chart folded to best place.
Put the hypotenuse of the triangle on your course
line.
Line up the center convergence point with a vertical line.
Read the degrees on the outside edge of the scale on that same line.
Pick either high or low numbers for either coming or going.
For me I add 15 to get magnetic and I have a course within one degree.
Easier to read than unfolding the chart and finding a rose then walking something over.
I haven't tried it on a boat yet but it looks promising.