Anybody who tells you what thermal mass to use without having used the same exact equipment as you cannot possibly know.
Every mash tun has a different thermal mass.
The way to truly determine your thermal mass if to back track some results and determine what your mass is. For instance, if you use 0.30 but are consistenly 3 degrees high, bring up your mash schedule, back the thermal mass down until the temp ProMash shows is the temp you are achieving. This will be the known thermal mass of the tun, and will be one you will want to use in the future. You do of course need to do at least one mash to determine this, and the 0.30 number we provide is just a starting point for the most common type of cooler tun, a 10 Gal Gott. Well, there is no single correct answer to this question, but I can give you some pointers.