In something as dynamic as racing, its almost impossible to put definate rules as far as passing/ blocking goes.  There is NO sure fire 100% way of determining it.  Its not a math problem, there is no one answer to it. 
 
It's not that hard, don't drive like an idiot, use your mirrors, show respect to your fellow drivers. 
 
If you need a "Rule Book" to tell you how many "moves" you can make, or how much overlap counts "proper overlap" then you should press SHIFT+S, watch, and learn.