Can't calibrate infrared motion detector
The problem is simple. These are residential motion detectors. Yours probably ran you somewhere between $20 to $30 dollars. Manufacturing costs on this was probably $7 - $8 material inclusive, let's say 50%. You can't expect a highly sophisticated piece of electronic equipment that you can calibrate to spec from these things. They are what they are, a basic low grade and inexpensive motion detector for general residential use. 

Many years ago we added these things in our no warranty list with light bulbs and ballasts due to the fact that most people were disappointed in their results. Even though they selected them and they were performing, just not the way they wanted them to. We were completely able to supply them with  a more sophisticated motion detector, but most people considered it cost prohibitive.

In short it's a cheap product and it performs in a cheap manner.