Bristol are correct - and to prove it:
To get the simple (normal) PNR time in minutes, we start with the observation that the distances out to the PNR and back are the same, as you are trying to return to where you started. However, distance is equal to groundspeed multiplied by time, so:
O x T = H x (E-T)
Where E is the safe endurance (i.e. allowing for reserves), H the homebound groundspeed and O the outbound.
Moving on:
OT = HE - HT
OT + HT = HE
T x (O + H) = HE
E x H O + H
All the formula does is find the ratio of the groundspeeds and divide them into the endurance, assuming normal TAS. For example, with 3 hours' endurance, and a 90 kt groundspeed outbound, with 150 home, the equation is:
180 x 150 90 + 150
27000 240
The answer is 112.5 minutes, based on a ratio of 9:15, which is actually the same as the speeds below the line without the zero
.