Hey Carl,
Glad I can help out.
You are correct. T-joint would combine 1 thickness twice.
You can't use thickest material for the HAZ control method because you have to pick which cooling curve to use (each curve is based on thickness of material). The simple t-joint cooling curve for 3 thicknesses looks far different than the single thickest cooling curve. Therefore, the preheat calculated could be way off.
The Hydrogen Control method is a much simpler method, in my opinion, where you can just use the thickest material.
When you have time, look in D1.1 - 2010, Annex I Guideline on Alternative Methods for Determining Preheat
Table 1.2 Minimum Preheat .....
Note "a" - tells you to use the thickest material.
Tyrone