i have 100,00 btu and 3 ton unit, i use 1 main trunk line, what should the size be? about, i now have 20 x8, i an confuzzed about what i see on line, looks like i need about 600 sq inches, is that right?thats about 30 x 20 rextnagle. i use 1 center return aera in the center of the house, thank you for you time,
richard