Use these ratings to decide your return line sizing based upon fuel pump output:
*
Up to 45 GPH = 5/16" or -04 AN
*
Up to 90 GPH = 3/8" or -06 AN
*
Up to 250 GPH = 1/2" or -08 AN
*
Up to 450 GPH = 5/8" or -10 AN
*
Up to 900 GPH = 3/4"or -12 AN
Use these figures as a fuel line sizing standard: if your feeding a carburetor with the typical 8-10 psi fuel pump feeding to the fuel pressure regulator
* Up to 250 HP = 5/16" or -04 AN
* Up to 500 HP = 3/8" or -06 AN
* Up to 700 HP = 1/2" or -08 AN
* Up to 1000 HP = 5/8" or -10 AN
* Up to 1500 HP = 3/4" or -12 AN
don,t skip READING THROUGH the links,
and sub-links theres a ton of related useful info in those