It is all a preference, and what the foothold will allow. I do know the grey padding of any thickness sucks, it will thin out and squish after not much ride time. The black stuff holds its shape. If the foothold is big enough than put 26mm black under padding top and bottom.
The best holds I have ever kicked into were on a superfreak with the black 26mm top and bottom. I couldn't even tell my foot was locked in because it was like 2 pillows pressing against my foot, but when I tried to pull out they were stuck in, it was AWESOME!!
And if you just ride flatwater, no need to pad the whole tray with the 26mm like the pic above, it just adds a lot of extra weight. Only pad the holds with the 26mm then do the rest with like 8mm black under padding.
Just my .02