I can't find the rule about the angle at which a conduit can enter a box through a KO. Is there one? It might have something to do with the need for a ground bushing because too much angle lifts one side of the lock nut (and connector) off the surface of the box.

Off the top of my head, I can't put a specific NEC Article on it, except 110.12 regarding mechanical execution of work.

However, this Article is basically unenforceable as it is 'subjective' as to "neat and workmanlike manner".

Your comment regarding the connector/locknut not being flush with the KO goes back to NEC Art 250 and a 'stretch' would be NEC Article 110.3 (B): re. mfg install instructions.

I'm not aware of anything beyond 'neat and workmanlike manner".

I thought it was a Canadian code or in a provincial supplement.

12-1008 requires at least 3 complete threads.

I would think that you're limited to going straight in.
Any bends would be made outside of the box.

I would really want to use a reducing washers but that really wont give you enough threads,#2/3 acwu i need to use a 3/4 throat wet connector into a meter base and I seem to feel more comfortable to use the reducing washers on the larger Ko;s on MB