To all of you who have replied to my original question: thanks very much. I am getting some great info.
I was reading some stuff from a RAAF training manual on medium level nav and would like to share a bit of mental DR for establishing position. The rule was developed I think, for ab-initio students flying the CT-4 Airtrainer (ie "Parrot"). Here goes:-
(Time of most current positive fix - Time of last positive fix) X 2 = Your current position in Nautical Miles from the last fix. The RAAF rule of thumb is that the "parrot" at that 130 KIAS (?) speed range with say +/- 25 KTS for headwin/tailwind: means that there is some leeway in multiplying by "2" (i.e. 2 NM/minute). This worked for me, when the instructor used to ask the dreaded question: "so, where are we?" Of course, there is the little caveat in the text that says that this will only work if you fly accurately.
Plastic Flyer After taking a 9000 hour 737 pilot flying in a Jabiru: "So what does it feel like to fly in an aeroplane that ways less that a 737's APU?