Water heater vent pipe incline

As a vent pipe exits the top of a water heater, is it ok for the vent pipe to transition horizontally at a level incline or should the pipe have a prescribed slope upward to rapidly exhaust the hot air/carbon monoxide?

Recommended incline is 1/4" per foot.

Also most manufacturers and standards require 12" vertical height from the WH draft hood to the first turn (elbow).