Yes, I made it up for the purposes of this discussion as I did not know what else to call it. I am trying to point to the 8/10 part (see table on p.25 of the Fellows booklet) - there is no mathematical reason it should be those numbers. They have arrived at them through practical experiment and landed on a standardised (or maybe commercially expedient) set.
Just had some time tonight to get back to this. I have a number of 1.25 dia cutters, so needed a way to mount them.
The key has been made longer so it uses some support from the slotted spacers either side of the cutter.
So I had to try it. Single cut with a 10 thou cleanup. Looks like I have some arbor deflection, but the supports are way wider than would normally be used..