Best posts made by Nuffe

@wilriker You can put the thermistor to a boiling water and measure the resistance. 100C will give you at least one accurate point.

@Danal Thanks! That helped.

Which documentation defines that clearly? https://reprap.org/wiki/G-code#G0_.26_G1:_Move says only this: "4) RepRapFirmware provides an additional 'R1' parameter to tell the machine to go back to the coordinates a print was previously paused at. If this parameter is used and the code contains axis letters, an offset will be added to the pause coordinates (e.g. G1 R1 Z5)."

In any case, now it works (it is very cool to be able to modify ongoing print...).