What equation shall we use to get the right friction factor for natural gas distribution low pressure networks. ??

I'm working with a software that it have included the calculation of friction factor for so called reduced pressure. The most important thing is to assure the right minimum pressure drop on the farest (longest) point of the pipeline. Then it calculates on each branch.

For low pressure0,05 bar, there are tables with unit pressure drop for each pipe diameter, starting with1/2" (3/8" not allowed), so unit x lenght of pipe = pressure drop. On the gas meter pressure drop1,75  bar, and it's not allowed to overpass0,5 bar per total. 

Regards,

Adrian Rimniceanu