They amount below the hole depends on which differential / transfer case being looked at.
We (I) don't know where the hole is relative to the axle tubes / diff gearing / cover volume --- the instructions may put the lube at the same relative level for each axle version
OM for 2004:
For 3 rear axles, 3 different fill instructions,
(for whatever reason, the 1500 rear is the only one doesn't use the traditional bottom of hole as the high end of the acceptable range for the gmt800)
The proper level for the 1500 Series is from 5/8 inch to 1 5/8 inch (15 mm to 40 mm) below the bottom of the filler
The proper level for the 1500 HD Series, 2500 Series, 2500 HD, and 3500 Series is from 0 to 1/2 inch (0 to 13 mm) below the bottom of the filler plug hole.
The proper level for the 1500 and 2500 Series vehicles with QUADRASTEER™ (4 Wheel Steer) is from 0 to 1/4 inch (0 to 6 mm) below the filler plug hole.
Add only enough fluid to reach the proper level.
Also -- 4wd front axle
When the differential is cold, add enough lubricant to raise the level to 0 to 3/8 inch (9.5 mm) below the filler plug hole.
And transfer case instructions say:
If the level is below the bottom of the filler plug hole, you’ll need to add some lubricant.
Add enough lubricant to raise the level to the bottom of the filler plug hole.
Use care not to overtighten the plug.
They must have had a reason for each one to have different instructions