I always find single flutes cut undersize. Never been sure it's a deflection or the tool. I always finish with a 2 flute and I also find HSS gives a better surface finish than carbide in aluminum for the finish passes.

I hardly use Single flute cutters anymore. My preferred method is 3 flute carbide rougher leaving 0.4mm then 1x semi-finish pass @0.3mm and a final 0.1mm final finish pass. For work that doesn't need the best surface finish and accuracy I just take the full 0.4mm as a finish pass.