You're right on the money there. I thought it was semi-common knowledge but maybe I was wrong. That's why it's important to pick the correct mobs to level on with your NPC. Pick mobs that are difficult but give a lot of exp? You might run out of time before you can reach your max kill limit. Pick easy mobs that you can kill quickly within the time limit? You'll reach your max kill limit too quickly and could have used that time fighting tougher mobs for better exp.
You need to know your capabilities as well as about how long/how many mobs your NPC will stay out for and adjust as your NPC grows in level.
Originally I started out in Bibiki Bay killing efts. Then I moved to Yhoator Jungle vs. Gob Bouncers and Hunters. Those lasted me until my NPC's late 40s. Now I don't level my NPC solo if I can help it. I'll get a group of 2-4 friends and start fighting DC-EM Raptors in Uleguerand Range (NPC is now 56). They provide a good deal of exp, aren't too bad in a group, and we can kill them at an efficient rate. At a minimum I can usually snag 1 friend to help, and I've found that strong EPs or weak DCs provide the best exp/kill ratio. *Mileage may vary by job.
Also I'm not sure if any of you who have done the Promy quest have noticed, but NPCs absolutely tear up empty mobs. My NPC was hitting for a minimum of 2x damage greater than anyone else. A friend's NPC was using Sturmwind for consistant 240+ damage (only on GAxe #2 as well). Next conquest tally I'm getting my friends I NPC-exp with and we're taking our 4 NPCs as high as we can get in Promy-Dem or Mea. I have a very good feeling about this seeing as how it seems NPCs have the strength of 2 PCs in Promies and their battle A.I. is improving at a frightening rate.