You know the depth of your pan from the pan rail, now hold a straightedge across the pump and measure to the pan bolt surface. I like to run my pickups about 1/4" off the bottom. To get the distance correct, move the pickup tube in the pump, make sure it fits tight when you are done. Many builders put a small weld on the tube to secure it to the pan, or run a strap from the pickup tube to the pump bolt. Don't forget to put the compressed thickness of the gasket into the measurement. I also will not run a hi volume pump with a stock capacity pan, you can cavitate the pump....an oil pan that holds an extra quart IMO is good insurance when running a hi-volume pump....