I've been sizing duct systems for years. I go by feet per minute. For square duct I figure the supply at 650 FPM and on returns I figure 450. That's what I was taught in school. I'm working with some guys now that like to use the static pressure method. .08 to .1. This makes a big difference. 2000 CFM at 650 FPM gives you about a 20x25. 2000 CFM at .08 static gives you about 20x15. That a huge difference. What's the preferred method?