I know this will sound terribly sarcastic, but it is likely that they designed the gearbox, and the casing, to fit the gears etc, then determined where the oil would have to reach up to to enable proper lubrication whilst moving. ANd then they 'marked' that spot and filled the case with it (or maybe they did it by trial and error) - and 'that' is when they worked out the fill capacity as such. but the capacity in and of itself isn't probably as critical as the oil level within the case. THere's no way they calculated the capacity by doing the math of the volume the gears would take up inside the casing, and what level that would have resulted in requiring what volume of oil,

Which kinda means that unless there's something drastically wrong with the engineers at datsun/nissan - then filling to the point it just starts to trickle out of the filler plug hole in the case (with the car up on stands or whatever, but perfectly level both side to side and front to back) is going to be the right level.

I'm assuming this is a competition car, so it'll be under higher load and see higher input (and output) shaft rpms than streeters will, and for extended periods) and I would be inclined to suspect that this is flinging enough oil (and creating a mist in some regions, and then add to it the fact that the air inside the casing is heating up and expanding, and taking some of this mist up and out as it expands and travels out the breather pipe - anyhoo - this is possibly not a sign of it being overfull. I wonder if extending the breather upwards a few inches (and going to larger diameter hose) might end up preventing it getting all the way out, and it'll seep back into the box as and when it can.

I have no idea what you might be running for the oil in the box, but if parasitic losses are an issue, then perhaps another pathway to reducing them is to look at some specialty synthetic oils (gl4 spec, not gl5, if I'm getting the nomenclature right - i.e. not hypoid diff suitable oil) - might be worth pursuing? Some of the automatic transmission fluids (for example) are more along those lines and have some sort of anti frothing additives (at least some of them do, and I have no idea which to be honest) .