The best plate thickness for the calculations done is in the 150mil to 250mil range, with the best value showing at 200 mil calcuation, which is 5,08mm btw
(note: the numbers were only run at 50 mil increments of plate thickness)
The calculations aren't for a 'specific' waterblock. They were done for a wb design of the generic nature being discussed, where there are water channels, there is an approximate 2"x2" active cooling cavity (where water flow and thus heat transfer actually occur), and an Athlon XP die was the heat source.
To do the calculations, one of the parameters needed is the film coefficient, which in turn requires a channel length. In this case I used a channel length in the range typical for the channel designs.
Below is another curve created with the exact same parameters except that instead of turbulent water flow through the channels, the flow is completely laminar. If you compare it to the other graphs, it shows why laminar flow in a waterblock is not a good thing