Really, you need to look at the dyno graph and see how fast and at what rpms the horsepower is falling off. For example if your horsepower on the graph is peaking at 5,800 but is a gradual curve down through 6500, you want to cross at about 6500. If the curve totally falls off at 6000 you want to cross at 6000. This will let the engine pull in it's highest power ranges throughout the 1/4. Your leaving alot on the table if you're crossing at 4200. My combo peaks at about 7,100 but has a nice broad curve that is still making over 400 rwhp at 7,500. To optimize the broad curve, I run a 4:56 and cross at around 7,500.