Conversions of Volumetric Flow Rate to Mass Flow Rate
To convert a volume flow rate to a mass flow rate you multiply the volume flow rate by the density of the gas.  For "standard" or "normal" flow rates, you calculate that density as though the gas were actually at your standard or normal conditions.  That is all there is to the conversion.  You don't need expansion factors or any of that stuff.  That was already included in the arithmetic used to calculate the volume flow rate you are starting with.
To get the volume flow rate from one base to another, you convert to mass flow rate and then divide by the new implied density
By the way, none of the "standard" or "normal" pressures or temperatures are actually standard or normal.  Every contract, every jurisdiction, every manager has every right to require volumes to be reported at whatever pressure and temperature makes them happy.  15C is often used for a temperature, but 15.6C is just as common.  101.3 kPa is often used as a base pressure, but 100.8 kPa also shows up.  In the spreadsheet you are working on you should accept a volume, a volume unit (MMSCF/d, m^3/hr, or whatever), Specific Gravity, an input pressure, an input temperature, an output temperature, and then calculate the mass flow rate and the converted volume flow rate.  Any assumptions of "standard" or "normal" will lead you down an stupid path.

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