The way I fill my gear box is I crawl under the car to remove the fill plug. I stick a very long tube into the fill hole. I then run the tube up into the engine compartment and push it onto the conical end of the gear oil bottle. When I am making a great big mess all over the driveway, I know the gear box is full. I button her up, throw some kitty litter on the driveway, and go.
When the smell of gear oil burning on my exhaust pipe seems like it is getting less, I know it's time to top her up again.