I usually count revolutions. I have to do it the most on my long arm because it doesn't have a numbered dial either. What I do is start at a known constant - for the LA that's the post being flush with the knob. I find the tension setting I like and count how many revolutions or partial revolutions forward or backward from that "known constant" and make a note of it, just like when I have a number, so it looks like this (just an example, I don't remember what the actual is at the moment):
Aurifil - 2/3 turns tighter
vs
Aurifil - "4"