Altsoba:
I'm completely new to this myself (only have very limited experience boxing WoW in the past), so I may be talking out of my ass here.
But if it's only one dot you need 100% uptime with, why not just put the dot in a step of its own in the dps key rather than the ability key?
Step 1 dot, step 2 all your dps abilities you want to use and put a delay on step 2 to not pass back to 1 for 21s. This will ensure that every 21s your dot gets casted.
Because while you have it in the same step as your other abilities, the delay in the ability key itself doesn't matter since at exactly every 21s you might fire off Shock instead for example, thus causing the delay and loss of uptime.