shorty79
The only rule of thumb I'm aware of is the one for the radio horizon. It is part of the JAR instrument rating syllabus and mentioned in notes 1 and 2 following 4.1.5.2 of ICAO Annex 10, Volume V:
D = 1.23 * sqrt(h)
D = 1.23 * (sqrt(h)+sqrt(H))
D: distance [NM]
h: height of the aircraft [ft]
H: elevation of the station [ft]
Your instructor's recommendation sounds familiar. I believe every instrument student has heard it and you'll see instrument flying is very much about getting and staying ahead of the aircraft. You can see from above formula that during your training over southern England reception range will normally not be a problem. 6000 ft and 120 kt will leave you almost half an hour to get the ATIS before you arrive at the IAF. However, you mustn't forget the ATIS is usually updated every half hour, so very often it will be a good idea to wait another two or three minutes and get the latest message.