I have seen more people put these back on more tight then loose. Here is what I have done with any bearing type axle.
Tighten the nut while spinning the wheel/drum/rotor. A good grunt won't hurt it, but don't hurt yourself. Back the nut off a turn. Spin wheel/drum/rotor a turn then tighten nut by hand without hurting yourself. When the nut becomes hand tight, if the slot in nut aligns with cotter hole, you are done. If it doesn't, loosen the nut to align hole with the last slot in nut. Grab wheel/drum/rotor and give it an in/out pull. If you feel a lot of slop (clunk, clunk) too loose. Just a smidge and I mean very very little, good to go.