Starcraft II - How to drop MULES most efficiently?

In this particular scenario, you should be manually dropping one on each closer/higher yield patch.

Worker/MULE AI will have the gatherer go back to the patch they were previously working on so this will help you determine which one they are mining from.

For Example you have mineral patches 1,2,3,4,5,6,7,8 with patches 2,4,6, and 8 being the patches with more resources and closer. Hotkey, click on 2, Hotkey, click on 4, etc. If you click spam on one of the mineral patches the MULE will simply go to the next closest one that is not occupied by a mule. If you were to just mass click on 2, the next MULE would either go to 1 or 3. The MULE after that would go to 1 or 3 (which ever one the 2nd MULE is not at), the MULE after that, the 4th MULE would go to 4.

It could also be going to 4 first and then 1 and 3 after. It's been a while since I've played Terran to see the exact patterns of movement for the MULEs.