Getting Perfect Miter Edges on Purfling

I learned a new trick today from Robbie at LMI for getting near perfect 45 degree edges for near perfect 90 degree purfling miters. I'll cut to the chase: He polishes the back of his chisels to a mirror like shine and then uses them as a mirror to align the chisel to the desired 90 degree angle.

See below:

Yep. Been using this since the dawn of time. A perfect mitre is one you cannot see.... In the image about, I can see those which tells me either they aren't perfect or the back of the chisel isn't flat. Part of the technique of making a mitre is to 'spring' the joint. In order to do so, your mitre has to be slightly less than the inclusive angle... I made hundreds of boxes using this principle. The angle was about 89.875 degrees