Endurance ( time ) x G/S Home
TAS x 2
Endurance 'should' include some buffer for you to hold at destination.
G/S OUT + G/S HOME = TAS x 2
eg. :
Total fuel endurance overhead or abeam to yr return airport = 3 hrs 25 mins.
If TAS = 300 Kts and W/C = + 35 Kts (ie 35 Kts tailwind component)
Then I would calculate PNR as ( with 1 hour hold buffer at return airport ) :
2hrs 25 mins x 265 =
300 x 2
2.42 x 265 =
600
1.07 Hrs. = 1 Hrs 5 Mins from your check-point.
Hope it helps.