Let the system cool to the same temp as you started and remove one of the #2 vents on each end and time again. If the time is the same just keep the extra ones as replacements for the the ones you are using. If you want you can then remove the #1 vents and time again. If no change leave them off. The benifit of leaving extra vents is if a vent clogs you have back up, the bad thing is if one clogs you do not find out until two clogged.
If your back-pressure is at 2 ounces as the system is chasing the air out of the main vents, then you are just right if you remove one of the vents, and the back-pressure goes up to maybe 4 ounces, then you know you need all the venting you have. You can never have too much, and the slower air velocity escaping is less likely to pick up some debris and plug the exit hole, when the venting is ample.--NBC