Snug down the nut while turning the wheel. Snug down and back off about a third of a turn a few times. After the last snug down, back off about a quarter turn and stick the cotter pin in. You should be able to feel a very slight amount of play in the bearing by wiggling the wheel. If you want to get real technical like we were supposed to do when I worked on trucks, we had to measure the play with a dial indicator, of course nobody actually did it, we checked the play by feel. With a castellated nut you don't have much choice, it's this notch or the next one. You just don't want pressure on the bearing, one or two notches back from snug on the nut should be good. When you're snugging down the nut it's also seating the bearing on the axle
. Try one notch on the nut, then another, see how it feels, bend the cotter pin and be sure the cap won't rub on it when installed. Here's another suggestion, when I grease the bearings I put a gob of grease in the palm of one hand, then press
the wider edge of the bearing into it to force the grease up through the bearing, work your way around until you have the whole bearing greased, do the same with the other one. Wipe some grease into the cavity in the hub. Be careful installing the seal so as not to distort it, and I assume you have checked the seal on the axle
. Slide the hub on as straight as you can so as not to damage the seal, install outer bearing, washer if there is one, nut, and do your adjustment. Take your time. Use as big a cotter pin as will fit in the hole.