It amazes me the number of people that "jump the shark" on this question, every single time.
1) I'm an American....I was raised with inches and yards. I know exactly what that shit is. I know what it looks like. I'm not measuring anything in cm or meters. I don't give a fuck if it is easily divided by ten.
2) Similarly, it matters not one fucking bit if I know that 1 MIL at 543 meters is 54.3 cm and 1 click equals 5.43 cm. Because I can't fucking divide my 16.73 cm POI offset by 5.43 in my fucking head anyway. So guess what? I pull out the goddamn calculator anyway.
3) If I'm calling hits and misses, I'll either call in angular units (MILS or MOA), or target relational units (1/2 target etc...)
4) If I am zeroing, or truing data and have to shoot paper, you can be goddamned sure I'm gonna put a ruler on it. You can be equally sure that some math is gonna get done. I have a $5 calculator to back up my smart phone, and I ain't afraid to use it.
Failing both of them, I can still do it long hand with pencil and paper if need be....because I'm not a FUCKING RETARD!
There is a reason we say use the reticle, but that reason HAS A FUCKING CONTEXT! If you can't make your students understand that, it is a testament to your failing as an instructor. If your students are simply unable to get that difference, then they are too stupid to do this. End of story.