It's easy enough to very accurately calibrate a tension meter.

All you need is a tree, friend, rope, sample spoke (or a few), and some cheap hardware knickknacks.

Basically, you make a tree swing, with the spoke in the middle. Ideally, your friend should weigh close to the tension range you'll be measuring, ie. 100kg.

If you have the original conversion chart, the one calibration will serve. Otherwise, you'll need to make charts using 3 weights (high, low, middle) and various spokes.