Your experience is not odd, Brent. You generally see it stated that the delta is "usually around 0.3" or something like that. Any examples of measured data sets I've seen shows this displacement is really all over the place, from maybe 0.15 up to the 0.7 range you saw. There is no simple conversion formula, as every mash is different, depending on grist, water chemistry, temperature of the sample, market trends in Bitcoin futures, and feng shui
. That's why, since pH in brewing always refers to pH at STP reference conditions, that's where you measure it. (And because that tiny thin glass bulb of reference solution really hates extreme temps. I killed a couple of expensive probes really fast by trusting a conversion formula Palmer published, thinking it would be neat to just dip it in the mash. And the formula was always wrong.)