Ok every rear is slightly different. usually 2 to 2.5 quarts. The way you check the fluid level is the fill / site plug in the housing passenger side casting.
3/8 socket wrench female square plug... unscrew the plug. fill the car on a level surface( I know it is hard to do with out a lift.) When the fuild starts to dribble out of the sight plug it is full ( gee... that was easy and kind of stupid proof....lol.
) Really that is the way it is done. When it starts to back flow, put that plug back in and you are done. Wipe it clean and have fun. Now you can tip the car slightly up on the passenger side and add a bit extra...NOT alot extra just a little more that will give the axle bearings all they need on turns . More great info can be asked for at wwwjdrace.com GM rearend specialist free registration also. Check it out for more rear diff questions. Thanks, Jim