I know this is old, but I don't follow your maths...

You said the best rewards are tier V, when tier I gives 2 instead of 1.6, then you say Tier II is better than VI, when 1.25 is more than 1.2, am I missing something?

I usually do Tier V if I'm lazy and Tier I if I have time (doing 400 will give you 8 rewards )