Vehicle cover saves

There has been a ton of debate about vehicles and cover saves. A ton of time and a ton of research for the answers. The rule book does say that the rules are subject to your gaming group but if someone was to join in, wouldn't you want to be on the same page. I thought i had a good argument and i was only proven wrong. Whenever a vehicle is in true cover, and it covers more than half the vehicle, that vehicle gets a 4+ for the save. It must have true cover though. No imaginary bushes, trees, and a mile high pile of dead Orks. Space Marines. This where what you is most important. Now what you disagree with how much is covered? i would go as far as measuring it. But that would be too anal. So you go with a 5+ save instead. That way you get at least something. What if the side your shooting at is fully covered but you can see the side in plain sight? Since you will be shooting at an awkward way and the side you should be shooting at is not possible. The vehicle gets a 3+ save on the armor that eing shot at.

Hope this clears things up a bit. If I am wrog then reply back as to why I am wrong. We are all here to learn.