Socca, I couldn't resist reading your thread, with a name like that!! Trig forever!
My experience is that Bubbles technique doesn't work! Now I'll explain why! Sorry Bubbles! It does work with a consant fuel flow scenario, but with, say, a normal cruise out, and an abnormal cruise back, it gets a bit messy.
A method that is easy (after you've done it a coulpe of times), accurate and will work for any type of PNR (3 eng, 2 eng, Depress) and at any "out" condition is as follows:
1. At a suitable point after TOPC, ie the START POINT, note the distance from the departure airfield (assuming that's the one you'll go back to if you have a problem). Note the fuel on board.
2. From the fuel on board, subtract the Fixed Reserve (30 minutes Depressurised PNR, 10 minutes Engine Failure PNR).
3. From the remaining fuel, divide by 110% for an Engine Failure PNR. To be super technical, we could assume that there would be little variable reserve needed for the OUT segment to the PNR, but to err on the conservative side, we take off 10% for the whole of the remaining fuel.
4. Calculate the SGR OUT by dividing the Fuel Flow OUT by the Groundspeed OUT.
5. Calculate the SGR BACK by dividing the Fuel Flow BACK by the Groundspeed BACK. This will be at the BACK configuration.
6. Calculate the fuel required from the START POINT back to the ORIGIN (at the SGR BACK).
7. Subtract the fuel required from the START POINT back to ORIGIN from the fuel available.
8. Divide the fuel remaining by the sum of the SGR OUT and the SGR BACK. This is the distance from the START POINT to the PNR. Add the distance from the START POINT to ORIGIN and that's the PNR from the ORIGIN in miles.