What monster power should you be farming paragon experience at? Well, that depends! It will depend on whether you are in a multiplayer game and if so how well you and the other players are geared.
Paragon leveling is all about speed; primarily killing speed and travel speed. You want to test an area to see how high you can set the monster power to while still being able to one shot the weak or normal white mobs. Note that some of these trash mobs will have a good chunk of health - you're looking for the normal, every day, run-of-the-mill trash mobs.. Not super weak, like the scorpions in Act 3, and not the tougher Savage Beasts or Trees in Act 1. Just the normal mobs.
Once you find the monster power that you are able to one shot stuff at, do a few tests runs through your favorite map. Take note of how much paragon experience you have before you start the run so you can see how much you have gained. Take note of the time as well or use a stop watch so you can keep an accurate track of how long it takes you to do the run.
After you do this a few times, you will want to lower the monster power 1 level and run through the same route on the same map that you just tested. As you do this you will want to keep track of your paragon experience gained and the time it took.
Take the average of time spent to paragon experience earned for 2 or 3 different monster powers, after you have found how high you can go while still being able to pretty much one shot the regular trash mobs. Then you will know what monster power you should be farming with your current gear, build, and class in that patch version.