Fill your external system, cooler, hoses, etc. and then drain back into a clean container, Measure amount. Remember (I wrote it on my work bench for both cars). Add to the fill amount in the future.
Never run new gearbox (full of oil from vendor). IMHO I would drive it around a few mins and then drain it and throw it away just like a new engine startup. Why? Because who knows what's in there from the factory. Low load running for a few mins will give any debris a chance to migrate to the drain plug area of the box and be expelled with the rest of the oil when you drain it.
I just did both of my cars, Always in January, engine oil, gearbox, coolant. I changed the brake fluid flush to July to do it when the humidity is lower and air temps are warmer.