Right, I'll try again.
I have tried to lay out the equation as it would be written, but could not get the system to work how I wanted.
So, in longhand: first, add gs out to gs home. Call the result gs mean.
Multiply the endurance (in hours) by the gs out, then multiply that result by the gs home. Then divide the result of that by gs mean.
The result is the distance to the PNR, from which you can find the time to PNR should you so wish.
To make my original effort mathematically correct, it should have been thus:
Distance to PNR = (Endurance x gs out x gs home)/(gs out + gs home)
Looking at it now, it's obvious that my original post did not make things any simpler, which was my intention. What's that about paving the road to Hell?