Hi Ian,

It's *definitely* not backlash - how do I know? Well I make sure not to change direction on the leadscrew when aligning all this up ....ie to set this all up, I initially I shoot past my registration mark, then bring it back (therefore take up any backlash slack) ....then use a 'G0 Y -150' command all travelling in the same direction to move to my other registration mark.

I've since learned about the G51 'scale' command, so I reckon I can use this little procedure

Set my steps per unit to mate with the leadscrew pitch (therefore 1.5mm @ 1/2 stepping = 266.6666666666667 steps per unit)...always leave it on this (I've been adjusting the steps per unit to dial out the pcb registration mark aligment errors)
Align over my first registration mark - zero the DRO.
Use a G0 command to move the Y axis 150mm (this being the distance between my PCB registration marks)...it won't totally align
Now jog the Y axis until it does align
Enter the actual DRO reading (eg 150.7 etc) into a spreadsheet ....the spreadsheet calculates the 'scale' adjustment
enter scale value as a G51 command.
Once finsihed clear the scaling with a G50 command.