When should you do a mission to reset farm code?

I have been doing alot of Shiro'ken Farming at Harvest Temple and have definately noticed a HUGE difference in the number of drops after a do a mission. My question is: Should you do a mission to reset the farm code after you notice a reduction in the amount of drops or quality of drops? After a while the number of drops, good or not, diminishs. What is more important to look at quality or number of drops? I only ask because I want to unlock as many mods as possible in the shortest amount of time.

Thanks in advance for responses.

i have no idea but trolls, minotar and other places that are over farmed seem to have low drops anyway...

I dont think this is the case with Shiro'ken Farming yet. I still get lots of purple and blue weapons,but not many golds.

if you care only about unlocking as many mods as you can just start pvp lol, is it really the mods you care about ?