Similar to Point of Equal Time or Critical Point (CP), the Point of No Return is a straight forward equation easily resolved on your Dalton flight computer.
The Point of No return (PNR) depends on the fuel endurance time. After passing the Point of No Return the remaining fuel will not be sufficient for a return to the point of departure. The flight can only be continued to the destination or to an alternate airfield. It is, of course, also possible to calculate the Point of Safe Return based on the available fuel after deducting the required reserve.
The simple formula for PNR is:
T (PNR) = E X GS Home/[GS Out + GS Home] (Time formula)
Where: T (PNR) = Flying Time to PNR & E = Endurance.
The formula can also be transposed as:
T (PNR)/E = GS Home/[GS Out + GS Home]
Example: Endurance 6h30min. GS Out = 240kt. GS Home = 210kt.
On the flight computer:
Align 450 (240 + 210) on the inner scale with 210 (GS Home) on the outer scale. Read 182min outer scale against 390min (6h30min) on the inner scale.
Result: The PNR will be reached after a flight of 182min (3h02min). If the point is to be located geographically, determine the distance to the PNR by means of GS Out. In this example the distance to PNR will be 728nm. (182 mins @ 240kt.)