Exploring putting XT L/S crank arm (with power meter) on Ultegra crank.

This would offset L/S 12mm more than R/S. I could add 20mm pedal extender to R/S, this would leave R/S 8mm wider

Any experiences dealing with off set Q- factor like that?

I have read on another post that 6mm on some bikes out of box is normal.

thanks in advance