Runner Diameter sizing is directly related to the wall thickness of the part.  Typically standard cutter sizes are utilized to machine the runner system, select the closest standard cutter size to keep regrind levels below 25%.

The runner that feeds material to the gate and part cavity should be 1.5 times the wall thickness for  Semi crystalline materials.  For Amorphous materials add approximately 40%.  For each branch of the runner towards the Sprue add .025 (IN).  The O dimension of the Sprue should then be .050 larger than the main runner.  The nozzle DIA should then be .025 less than the O Dimension.

General thermoplastic materials with no fillers,  and their typical runner diameters.