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That's the real question. And there is the bit about what's going on the the old grease in there.
As far as how much to add, best I think I've read was to fill it from the rear (highest) bolt hole until you could just see it in the front bolt hole (about 1/2"-1" down). Leaving the third bolt to hold the cover in place.
Or fill from the read bolthole and use something to stick into the hole to check to see if it's down about 1-1/2" but the grease will need to settle some for that to work