I've just copied this from another thread...
Shortly after I joined the airline, I was in a class in which in flight calculations for diversions, etc, were being discussed. I made a comment to the effect that I could work out the answer accurately, and covering many more of the variables, simply using a chart and a piece of thread. Many years later, when the bloke who was running the course was a Classic Captain, I flew with him....and in the middle of the night, out came a chart and a piece of thread.
Can any of you work it out? You know where you are. Where you want to go. The average wind. The average true airspeed. Average fuel flow, required reserves, and average fuel flow.
The same basic technique will give you answers for point of safe return, and point of safe diversion to both and on track and off track diversion points. And it's really simple.....
Actually, a couple of pins help too, so if I ever have to put out a call on the PA for a sewing kit, then you'll know what I'm up to.