I think a good working trot is the trot the individual animal can do with least amount of effort. With each of my horses that is different. I think this is one reason why when you buddy up with another horse at a ride you may find your horse does not perform as well as you expected because you can be altering what is most efficient for your horse in order to stay with another rider.
If you like to use gadgets, you can use a GPS and a HR monitor to find out what the most efficient speed is for that individual horse and different types of terrain (meaning hills, downhills, flat, sand, etc.). If you don't use gadgets you can use a measured course and possibly recovery rates (although I am not sure if the information will be as accurate as an on board monitor). Of course, this is only indicating what is most efficient cardiovascularly and does not reflect what is best for the animal's physical structures like legs, tendons, etc.
When my mare completed her first 100 I did very little walking because the mare's most efficient trot was slow enough to trot through 99% of the ride. It made pacing for the ride very easy.
Instead of looking at working trot in terms of an exact MPH number find out what is most efficient for your horse and use that as part of your ride strategy.
