First of all, please note that there is a difference in datum used for the two parameters (which is probably not an ideal design.) GT is expressed as meters between MLLW and MHHW. The salt elevation is expressed as height above MTL (the mid point between MLLW and MHHW), so is a lower value than the GT generally.
As you have likely seen in some of our reports, this is based on "frequency of elevation" analysis from observed-data analysis so that it takes into account wind tides as well as astronomical tide cycle data. I can point you at one of these analyses at some point if you'd like. If you calculate a daily high water level using NOAA data for several years and then calculate the 0.967 percentile of those data that pretty much will get you there.
Best! -- Jonathan