Updated first post with findings. Revised formula
Weapon/Amps Efficiency Formula
Efficiency=(x-2)(23x-42)/2
x=max_dmg/cost (maximum dpp)
All but verified by MA
Economy enhancer does not adjust efficiency rating
Accuracy enhancer does not adjust efficiency rating
NOTE: As the relationship between efficiency and dpp is nonlinear, you will get different results if you try to calculate efficiency from a combined dpp item. Combining efficiencies is calculated on cost (decay+ammo), compared to dpp which is calculated on dmg*eco
An item with max DPP of 2 or lower will have efficiency of 0.
the max attainable economy rating happens at 4.86DPP (Impossible without an items already at 100% efficiency)
Interesting findings from this
I suspect the Unique Legend War axe and imp 7 knife have incorrect dpp's in entropedia.
Test: Looked up Mann MPH DLXe in auction, which has a very close dpp in entropedia as ?Unique Legend
From entropedia:
Mann MPH DLXe=2.174
Unique Ax=2.164
The implies efficiency for Mann MPH DLXe should be 0.7% (correct, confirmed in auction)
The implies efficiency for Unique Axe should be 0.6% (incorrect, auction states 0.0%)
Economy enhancer does not adjust efficiency rating
Accuracy enhancer does not adjust efficiency rating
Conclusion:
Displaying efficiency is a good thing! This prevents discoverers of unique weapons from lying about economy, as we know the conversion rate.