Auto Gearbox Fluid Change Issue

[tricky1962]

My trip around the block is up quite a steep hill initially so I wasn't surprised the fluid was warm.
Among other things, I work daily with liquids between 25 and 50°C so believe I am a reasonable judge of liquid temperatures in that range.
What I plan is to top up the fluid when I get my next lot delivered, run the car at idle on my drive until the coolant gets up to temperature (which could well take 15 mins), the idle speed drops and then check the level. I have a thermocouple so I can check the actual temp of the oil as it comes out.