rear end diff fluid level?

[71RS/SS396]

I fill mine up until it comes out of the plug on the side of the 3rd member (approx. 3 qts), install the plug, fill some more, spin the drive shaft to fill the cooler, and remove the side plug and drain it down to the level of the side plug. Fwiw I've been using Mobil 1 LSD gear lube with a tru-trac with no issues, I was killing gears when I used non-synthetic lube.