I just squirt 90-140 into the center of the hub before sliding the outer bearing on, by the time you adjust the bearing and spin the wheel/hub several rotations, the bearings have as much lube as they can hold. The seal should not leak even if the hub is 100% full since there is no pressure built up, they excess lube will drain into the axle tube.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy