Their design doesn't look particularly amenable to tuning though. I'd imagine some design with the weight on a screw that can be screwed in and out to adjust the centerpoint.

The two bucket design doesn't seem overly problematic though. Often you can see this in the record as time-to-tick will follow a sawtooth pattern. I haven't tried it myself, but .. assuming you don't have any skips in your record.. I imagine you could periodically use a long syringe and measure the exact volume that triggers a tick on each side. And then correct the record in software.

A bit of a tangent.. but I've also not found any software or algorithm for correct interpolation of rain volume. Maybe you know of something? ChatGPT suggests a few things :)) but I wonder if there is some standard method people use