I just did this job for the second time on my 1999 ML 320 which now has 117K miles. I do have the special tool and an infared thermometer. This last time I did put the magnet in the pan, although there was zero metal, shiney particles, or slug.

What I do to make very little mess is as follows:
-drain torque converter first and hang a 5 gallon bucket underneath converter and keep it as close to converter as possible so no splash.
-put bucket under transmission pan and drain it out.
-put drain plug back in pan and let it sit overnight.
-remove plug and drain what is left.
-put drain plug back in pan and carefully remove pan, it will still have fluid in it.
-pour into bucket, and then measure total fluid removed, mine measured a couple of oz. short of 7.5 liters.

I put in 7.5 liters and with temp of trans fluid was at 28 degrees C with the engine running after 3 mins. On the special tool it measued about 1 mm below the second mark for what temp should be with fluid temp at 25C.