Simple answer: Use logic.
You want to do as much damage per second as possible, so your highest damage per activation (DPA) time attacks will be used as often as possible. The highest amount of recharge any Scrapper can reasonably achieve (unbuffed) is about 270% for perma-Hasten. Figure out your recharge for your highest DPA attack then squeeze as much damage in the recharge time as you can. Don't forget to include Arcanatime. Look it up on Paragon wiki if you need to know how to calculate it.
Spreadsheets help a lot.
Mids' can now report Arcanatime as activation time, making calculations even simpler.
"That's because Werner can't do maths." - BunnyAnomaly
"Four hours in, and I was no longer making mistakes, no longer detoggling. I was a machine." - Werner
Videos of Other Stupid Scrapper Tricks
How is it done?
Is there a formula or something more elegant than trial and error?