Spotting and loading is a question of speed and being able to drown out the background noise. It helps if you can do the math in kilos, too. But I always have a sheet with colours. I remember calling the plates for Meanna Franco's bench press record in an IPF meet a few years ago. I called what was on the screen, she lifted and it was successful but apparently it was miss loaded. The computer was wrong, I was right, the spotters and loaders were right. You still feel shitty though. Also back spotting the squats is stressful the first time. You don't want to jump in too soon, but you also don't want a dead lifter.