Ya but you can't test with .20s because the energy gain isn't linear
so it has to be with what they're saying they'll use
So either they lie to you about the weight they have in their mag, or you have several different weights of chrono BBs and they pick a different weight anyway, or the event has to supply all BBs of every weight to everyone and they go trade with someone else.
The only way to ensure nobody cheats the chrono is to assign ammo to people and then randomly spot check them on the field, or have everyone run the exact same weight and have special colored ammo (like orange) so it's easy to spot.
Whether you do the calculation on your phone, on the chrono, or write out all the weights on the chrono, it doesn't affect the end result, just how fast you get to it. If they want to cheat the system then they're going to try and cheat the system.