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.