"Then 13.7 it is.  So when running 200F LoP and 15 gph, I am cruising
235-240kt TAS on 59% power?  I get more efficient with each iteration!"

I figured Walter would have jumped in here by now.  All the previously
listed numbers could be correct.  It all is based on the brake specific
fuel consumption (BSFC)of your specific engine.  Some engines are better
than others.  Carbureted engines usually have higher BSFC numbers
(poorer fuel economy).  You need to get data for your engine and then
the "factor" works LOP.  Note.... LOP not rich of peak.  The BSFC at
take off power is much much higher than LOP cruise.
Craig Berland