When do you guys stop calibrating mods?

So for your first question, obviously it depends. Decent mods will be calibrated once. 

Very Good mods, or good mods with very high potential will be calibrated twice, and superb mods 3rd time (the 40s).

For reference - superb mod is 22 speed with 4 speed rolls.

As to you second question - no. the odds to get it better are so low, so I never try

I didn't even know there was a 150.  I do 25s if the stats are really good.  But not too often.  40 would  be a stretch because I can always get more mods.