It'a just trial and error, sometimes my mirrors fall out of my preferred position, and if it's the left one, I can slowly fix it while riding, pending no traffic around. But basically, get a car behind you or go on an empty road and imagine which angle would be best to see if there was a car behind you. You're not going to get 100% visibility from your mirrors, half of it gets hidden my shoulders and arms. So you might have to weave your head or torso to get a better view of traffic. And most important, still keep in habit of head checking when in doubt.