I decided that the flash size is not sufficient to allow internal calibration logic and now resorted to external calibration with a script and a sigrok-enabled multimeter. This allows for very accurace Least Squares Estimation and gets the accuracy to be pretty much spot on.

I'm happy with the project as is even though I can see many improvements that are possible to it but for now it is sufficient for my needs and I have other projects to attend to so I don't think I'll work on it so much from now on.

I'll still appreciate comments and especially patches or merge requests if someone fancies to work on this further.