The TRV is the round's travel time, when the TRV is the smallest number, that's C0, the longest TRV number is C2, and the middle number is C1. After you get your call for fire and do your calculations, you aim at that point, usually a hill or rock, then you tap F to cycle through and watch your TRV. I choose a nearby point that is within range of the shortest range charge (C0), but also far enough out that I can hit it with the longest range charge (C2). There's gotta be a way, right? It doesn't make any sense that you wouldn't be able to tell what firing mode you're on. But this only works if you can see your target (which in 90% of cases you can't). So far the only way that I've seen that you can tell is by pointing your 'scope' at the thing you want to hit and it'll tell you if you can hit it, and then cycling F until you get the 'in range' thing to show up. I know you press F or whatever your change weapon key is to change firing mode (short, medium, far, very far, extreme), however I cannot for the life of me figure out where you're supposed to tell what your firing mode is currently set at without using the targeting computer or just firing, pressing F, and firing again until you actually hit your target (which is just a hugely unnecessary waste of rounds). I've been playing around with mobile artillery (Sochor and Scorcher) recently to try and learn the maths around artillery, so I've been avoiding the targeting computer for the most part.

Arma 3 how to change fire mode