Not sure if this will help in sizing your gas pipe but try the following:
http://www.combustionresearch.com/Infra-Spec/catalogs/PDF/EDS6-Rev_1.pdf
Specifically Pg 14, hope it helps, it is a chart vs a calculation.
You could try a Oil Fired Engineered Designed Systems - Vacuum Vented Radiant Heating Systems from Combustion Research Corp. They have another version that is powered verses vaccum. The other benifit is that the units are radiant verses forced air so you would get better effiecency from the...