100 + 28 degrees are not harder to mark than 64, and then aim 0 and 100 properly. :-/
One can build such a tool, but it's not a doubled-over piece of string.
I don't see how you're going to use this system to effect repeatable markings on glass tubes.
Try it: it's easy. Use a flashlight from your phone instead of a candle, then project light to a wall, then put your palm in the front of the light.
- get some string
- measure a length between your low and high points
- fold it in half
- make a mark at the halfway point of the string
- fold it in half again, etc.
No candles, projection, transparent or slotted ruler, wall, or carefully moving one's hand back and forth under projected magnification needed. Just some string.