Good way to do mainsheet marking?

I am looking for a good way to put markers on my Laser mainsheet. I have two markers on my main sheet: One at 90 degree boom angle and the other one for a little narrower angle. I tried (a) blue and red color sharpie, and (b) Scotch vinyl plastic tape, but neither one works well. With (a), the color fades out after 2 or 3 rides. With (b), the tape position tends to slip after 2 or 3 rides. So, I have to re-do the marking every 2 - 3 rides, which has been not convenient. I am wondering someone knows a better way. One way I am thinking is try colored rope whipping such as Your suggestions are appreciated.
A whipping would work, of course, but it may break the uniform-thickness (and material) rule, as pedantic as it may sound. The reference-point rule itself is more lenient. You'd have to check with a real measurer. Sounds good otherwise.

Black sharpie. Make the 90 degree mark big and then put your stopper knot at the 110 degree angle. Yes, you will have to re-do the sharpie mark from time to time.