I am also a bit familiar with US 81mm and 4.2" mortars. The "four deuce" uses "donuts" of powder charges and trust me, the one fire call you didn't want to get at your mortar track was "charge zero"! That meant that the bad guys were within rifle range of YOU!! Time to shoot and scoot!
I had actually forgotten about the donut charges for mortars until you mentioned it. I wasn’t a mortar man, but was in a weapons platoon most of my time in the USMC. As the assaultman section leader, during deployments I was a leader without men when my squads attached out. Because of this I headed up the company FST team and I trained with mortars a lot. My last deployment things had gotten bad enough with ROEs that We just got turned into another rifle squad, although I did attach out one assaultman to each platoon to do demo. Mortars were fun to work with. I was always amazed watching a good mortar plot board guy use the whiz wheel and spit out commands in a matter of seconds from taking the fire mission input. Lots of respect to professionals who did their job well.