Started to use 1.5 at depth because thats when the scrubber initially has the most life. Also, high po2 reduces deco obligations slightly, then dropping to 1.3 by by 1st stop and then 1.2 through the stops. The idea of this is to give lungs an easier ride on long decos and improve their effeciency. Ive done some 4-5 hour decos but didnt really notice any difference, but can see only pseudo science in the traditional routes (low po2 at depth/high in deco) and some at least academic benefits the other way round.