So this method is no good?

This is the method which I use and find it works quite well. (note pic, to follow).
First, the gearbox oil needs to be warm, idealy between about 40-80 deg's so take the car for a short run.
Now jack up the drivers side (passenger side if left hand drive) and support on a suitable axle stand. Note, the car needs to be on a fairly flat surface.
Warning: The Catalytic converters get very hot and the level plug is very close to them so be careful!)
Place a suitable container underneath the auto box and locate the oil level plug, slacken it off with a 13mm socket (you could use a spanner but, a socket is easier), dont fully remove it.
Now chock the wheels, ensure the auto box is in park and start the engine.
Carefuly reach under the car, position the container below the level plug and remove the level plug.
Using Dextron 3 ATF top the fluid level up until it just starts to run out, for top ups I buy a 1 litre bottle with the long extandable plastic delivery tube, insert the tube into the level plug and squeeze.
Now, with the engine still running, lower the car off the jack, the excess fluid will run out and into the container.
Jack the car again and refit the level plug with a new washer (if you dont have one then heat it up to re-anneal the old washer before fitting).