Due to backlash, only apply adjustment on the out-stroke of the adjuster bolts. A little bit will do, say an 1/8th turn at a time. 3/4" slack is a good target to hit. The indices mean little to nothing, as they're prone to error from what looks like a stamping, as opposed to being machined into the steel. A good rough way to get both sides equal is to count exposed threads on the adjuster bolts with a toothpick. A home-made or fancy trammel bar (tram bar for short) is perhaps the easiest way to achieve accurate rear wheel alignment, but that's a long story for another day.