@bluecrayfish In your config.g, you should have a line to define steps/mm, M92 with parameters for X, Y, Z and E. The value for Z should be 400 for CR10. However, it might depend on the position of M350 (microstepping) and the values you enter there. Here is a snippet from my config.g that works:
M92 X80 Y80 Z400 ; Set steps per mm
M350 X16 Y16 Z16 I1 ; X/Y/Z: configure microstepping with interpolation
In M92, I’ve left-out the E parameter because I use a geared extruder instead of the CR10’s original one.