Wish to add DRO to tube bender using rotary encoder and timing belt

Need some advice. I want to add DRO to the length axis of our tube
bender. I have seen something similar which had a long toothed timing
belt approx 3m runing thru a rotary encoder with a couple of small
wheels acting as tentioners. The encoder was fixed on a linear guide
that allowed it to run up and down the timing belt. I' m looking for
0.5 mm acuracy / repeatability (I know they're different). A similar
system can be used for the plain of bend angle but using a closed belt
measureing perhaps 0.5 degree. Where shall i look first?