you can save a lot of time by throwing in the first handful of marigold while the fire is going, it won't affect the outcome, and just wait 2 seconds for the fire to die out before the second handful (or get the extra screw-up perk and save those 2 seconds....)  You do not have to wait for the fire to die to toss in the first marigold with 0 feats, you will still make the potion.