The "0.35 correction" is really something of a homebrew myth, actually. pH will always measure lower at mash temp than the actual room temp pH, but there is no correction formula. The displacement is unique to each mash, depending on grist, water composition, time, temperature and other factors, and varies widely (from around 0.1 to 0.6.) The only way to know the mash pH is to measure it at room temperature.