I will try to answer your questions.
You said, “what information do I have about where I should install the compass for better precision”. If the location passes the “check mounting location” procedure then that area is fine. The compass will not give you a numerical indication from good, better, best, of where to mount the compass. You will only get a good, or no good indication. From my past experiences with the PG1000 and PG500 compasses if it passes the mounting location test, the compass will set up within an error of a few degrees at the worst. If you do have a slight error in heading, the most important aspect to check is to see if the heading is off an equal amount in the four points, north, south, east, and west. Then with the error information, set in a heading offset into the compass.
You said: “I have tried many times to perform the “check mounting location” with every kind of combination possible, the results is the same; I end up with the status led RED”. I agree that the procedure should be changed (I have taken steps to make this happen) so an installer cannot receive a false indication. As it is now, if you perform the test and wait about 2, 2.5 minutes before you either move on to the next test or power off the compass, you will always get the status light to turn red, it will do it every time. The first indication that you receive after starting the mounting location test, is the correct indication. If you get both the status and the check light to turn green first, then the location is good. If the status light is red and the check light is green, then the location is not good. Most of the time you will get an indication well before you rotate the compass twice, or even once. If it is a good or bad location this will show up right away, usually within a half turn or so. If you rotate the compass extremely slow (takes two minutes to make a rotation), then you could conceivably get an incorrect indication.