What approach do people have when considering the spread bet? For instance, Fed beat Wawrinka in 4 sets but didn't cover the -4.5 line (3-6, 7-6, 6-4, 6-4). Djokovic took 5 sets to beat Cilic but would've covered -4.5. But obviously, a 5 set match implies Djokovic struggled more than Fed in a 4 set match.
So is there a strategy to determining when to take a fav or dog on the spread? Or how to know whether you made a good bet?