Specifically, the FAA guidance to AME's.

Safer to let your AME do the math, and determine the pharmacological half-life, if any. You can do it yourself, and it will be fine as long as you do it right.

Again you don't have to wait for all traces to flush out of your system, like a pothead doing a pre-employment test. It's OK to pop positive if you have (and comply with) a prescription, and if the level detected does not indicate impairment. The DOT test is looking not just to see if you're high, but if you've been using at all. You're never allowed to be impaired on a DOT test, but detectable traces related to prescription medication is OK.