It's a function of ground speed and descent rate. So a 777 with a 50kt headwind is going to have a different TOD than a 777 with a 50kt tailwind. 

 

If you have an Android device, there's an app called Distance Calculator. You put in your current altitude, target altitude, descent rate, and ground speed, and it will tell you how many miles away from your target you should start your descent.

 

The pro version will also calculate required descent rate when ground speed, altitudes, and distance from target are known.

 

 

For a standard 3 degree slope you can work it out with a little simple math. 3 times your current altitude divided by 1000. Or for example 38000 feet becomes 38x3 = 114

So at 114 miles route distance from your destination start your descent. 

Many commercial jets will need somewhere between 1750 to 1850 feet per minute descent rate in Pilot2ATC, that's just from my experience. But of course a tail or headwind will make some difference also. For a headwind descend a little later and vice versa, I don't think P2ATC really takes that into account though.