I may be stating the obvious here, but if you are falling just a bit short I would try to stay at the higher end of the recommended power zone for the rest of the interval. Any more than that and you would be basically doing a different workout.
If i need to step off the gas for 5 or 10 seconds because of a stop sign or people crossing the street, I stop looking at average power and try to stay positive about the fact that NP will still be inside the prescribed zone. This is just my 2 cents though…