I meant I've written a few posts here on adjusting rod length, not delta radius.
M500 doesn't give an acknowledgement it should return "Settings Saved to config_override.g", maybe a quick fix for DC in next release?
I suspect your probe may have a 0.75mm offset, Piezo works at around 0.1mm but still a slight offset. Determine the probe offset by experimentation, by doing G30 at bed centre, then raise the nozzle 5mm. Jog down manually in small increments until you actually have nozzle to bed contact. Now read the indicated Z height.
The real offset is the difference between 5 and the amount you jogged.
So if you got down to 4.25mm then the nozzle touches the bed then the offset is -0.75mm.
Use this measured z offset for your probe offset in G31. Its positive if the probe triggers above the bed (IR/inductive), and negative if the probe triggers below the bed (nozzle contact).
Then calibrate and manually check height, If you are still getting the height discrepancy (you shouldn't be), then put
G30
G1 Z10
in your start gcode so that you "zero" Z height before each print. However, this still relies on the z offset for your probe being correct, and consistent. This is why Piezo and Smart Effector are so well received they are very accurate 10 microns or so, and very consistent varying no more than 5 microns (in a stiff, well built machine).
Edit, also see Adding trigger height corrections to the bed.g file on this page https://duet3d.dozuki.com/Wiki/Calibrating_a_delta_printer
