Shortly after the last update, everything was attached together into one board, sprayed with conformal coat, and installed into the scales. It's been in use for about 14 months now by my parents, and the backlight has been extremely appreciated. Battery reading according to the ATTiny is still above 4 volts despite regular use. It was calibrated by very simple y=mx+c, with the m factor being determined by stacking two scales together and standing on them both. The c factor of course is zeroed out by tare-ing.

It still lacks polish - zeroing is indicated by the appearance of a decimal place, and there's still some noise. However further testing revealed at least one source of changing reading was the human body's pulse - you bounce ever so slightly with each heartbeat. 

A memory efficient exponential smoothing with a long time constant solves most problems though.

Next steps are to document with photos, confirm battery reading, polish software, and maybe add a heartbeat mode. Checking the calibration has maintained is also important, though that's largely due to the strain gauge itself and there's little ability to change an intrinsic flaw like that.