This formula is for U17.5 and onwards and is still up to date as of U23.1
As some of you might have noticed, Duration mods now affect the efficiency of draining powers. The actual effect of those mods is that they increase or decrease the length of "one second of drain".
Here's a little chart of the total drain you could expect from varying combination of duration and efficiency mods :
The values I chose for abcissa and ordinate are not random, they are amounts of duration and efficiency you will get when equipping various maxed mods together. I did not however include arcane helmets.
As you might have noticed, I included 190% efficiency in the chart, and that's because the efficiency cap is not exactly 175% but rather that the total combined cost cannot go below 25% of the original cost.
The actual formula is :
Total Drain = (2-Efficiency) / Duration
For instance : when you have 135% Efficiency and 194.5% Duration, you do (2-135%)/194.5% = 33.4% Total Drain ( or (2-1.35)/1.945 = 0.334 ).
Bear in mind the Arsenal will not show when you have more than 175% efficiency, it's up to you to figure it out depending on the mods you have equipped.
Also, you can see on this chart that Fleeting Expertise doesn't always have a mitigated effect due to its +60%/-60%. This is only true when you have no other efficiency and duration mods equipped. It is true that the 40% duration 160% efficiency gets a total combined efficiency of 100%, but check the 40% duration and 190% efficiency, the cost went straight down to 25% and that's just adding a Streamline in the build.