I used to love reading Frank Berto and sort of enjoy seeing references back to what he wrote BUT, it seems odd to me that Jan and others always refer back to Berto as the source for this 15% guideline. Where did Berto get it from? If from his own testing, has anyone tried replicating those tests? Beyond that, how useful is this rule of thumb to the average cyclist anyway? How in the world am I going to measure that? If one tries to chart it, isn’t one going to need to do measurements for every single weight on every single model tire with every single inner tube (or lack thereof) combination on every single rim to get anything very meaningful? All this before one even thinks about the accuracy of the average pump gauge or how much pressure one loses removing the pump from the valve. With a floor pump, I pump to about 5 psi over what I think I like best and ride until it seems like things are a bit soft, which might be 10-20 psi or more below what I think I like best but, I really don’t know because I don’t know how much pressure I lose connecting the pump or how much pressure goes into back feeding the pump. With the hand pump, I use a thumb test combined with how energetic I’m feeling. Given all this precision, I think I prefer a 14% drop.