I find when carbon dosing I have to spot feed my LPS. I can't seem to find a middle ground between carbon dosing and not spot feeding IME. I do also feed several times a day so there is food present more often. Then again if I wasn't feeding several times a day, maybe I wouldn't need to carbon dose...
I'd be curious what is in that and if it is actually any better then other carbon sources like sugar, vinegar, or vodka. $15 bucks isn't bad, but if your paying $15 for 500mL of some vinegar with sugar added to it that would kinda suck. Not saying that is what Red Sea NO3:PO4-X is because I don't know, but vinegar, sugar, and vodka all seem to do the same thing. If you do decide to go with carbon dosing, make sure to remove your GFO. It will make things much more effective IME.
HTH