Gearbox oil level

After having a few problems with my C70 with the driveshaft etc, i replaced the drivesaft, but when i withdrew it from the gearbox a load of oil came out as well.

So i put the new one in and was going to fill the gearbox back up after locating the fill plug after some searching on the net, it appears that you have to remove the gearbox mount to get to the bolt head on, and assuming ive got the right one is approx level of the steering rack which is about half way up the box?

When i removed the plug another load of oil came out, so i let it come out until, it was the level of the hole, is this correct?

It appears if ive done it right the gearbox was overfilled big time.

Or have ive done something wrong?

The car must be level when the fill plug is removed.