I have commented lock-outs on various occasions
I think it is a fast, easy to use method of tensioning, and many LO's have excellent stringing-tables and clamps. But the absolute accuracy is horrible. Most users of a LO will tell you: it doesn't matter that the absolute tension is not known. If you want a tighter stringbed, just put the reference-tension up, looser, then down. You could better have a scale from 0-100, and work from there
Calibrating a LO is hardly possible ( in the absolute-value-meaning of calibrating). Same as with the actual stringing, the calibrating depends on type of string, speed of cranking. And, you would need an expensive measuring device that registrates the tension at lock-out. A simple spring-based-calibrator can not do that.
This means actually, that even two identical lock-outs will give different results if "calibrated" by different stringers. And again, this is not important if you only string for yourself.