I'll stick with my park tensiometer, but I do like the idea of building a calibration method. Sounds like a fun project. Maybe even as simple as hanging a known weight on a suspended spoke and comparing the expected deflection against the tool's measurement.
I've got a spoke hanging from the ceiling, and a bosun's chair that hooks on to the spoke. I'm the known weight, I get in to the chair, hook it up and I'm all set for a one-point calibration control. If I want another point, I'll recruit either a skinnier or a chunkier relative, neighbor or riding buddy. Any one ready to be honest about their weight really.