Spindle perpendicularity

I’m changing my universal HicWic tool mount to the mpcnc standard tool plate.
I have a hard time trying to calibrate spindle perpendicularity with Ryan’s tool (http://www.thingiverse.com/thing:1525661).

I have about 1 mm of height difference from one side to the other. Is this difference acceptable?

If needed I shim it up with some tape in the mount, but usually if is front to back not side to side. Make sure your gantry is perpendicular first.

You are right, my gantry was slightly out of square. Now it’s near perfect.
I’ll try perpendicularity of the spindle again.