8 3/4 Thrust Block

I've been reading about the 8 3/4. Specifically the thrust block used in conjunction with Mopar tapered roller axle bearings.

I don't understand how it is expected for the bearing rollers to be properly engaged in the bearing cup simultaneous with the tip of the axle pressing with the correct force on the thrust block. It seems that if the axle was only a few thousandths short the thrust block wouldn't do any good. Conversely, if the axle was a little long, the bearing wouldn't seat properly.

I'm plan on having my axle housing narrowed and from what I've seen about how it's done I worry about the necassary accuracy for this to work.

Am I missing something?