I just do it like I've always done, use a pump to fill through the fill hole until fluid starts to spill out, put the plug back in, done. I've never and have never heard of anyone having a problem doing it this way.

I also run full syn in front and back, 75w-90 front, 75w-140 rear with a tube of friction modifier mixed up in the first quart to go in each of them.

I drained/filled each diff when I first got the vehicle, part of routine maintenance for any used rwd/4wd vehicle I buy. That and the TC/trans fluid and transfer case as well...

edit: and you don't want to "top off" diff fluid. If its low and was filled properly its leaking and needs a new gasket or cover or both. If it isn't leaking it wasn't filled properly. Either way its best to just drain all the old funky fluid out and give your vehicle new fresh stuff that'll last you a while.