How do I balance air outlet/inlet valves without an airflow meter?
I tried this approach on my senior project.
It is a little tricky, but you find the largest garbage bag you can buy, or any bag, and either a stop watch or better a video camera.

If you know the size of the bag 50 gallons in my case.
And if you know the time it takes to fill the bag. 

you can calculate the airflow through a particular outlet.

The bag can fill up pretty quickly, so I recommend making the calculations using a video camera.  It helped to see when the airflow initially started.

I had a known fan that produced a airflow speed of 150 CFM, and I was able to measure out about 133 CFM with this technique.

The video camera is used to see more accurately when the blower starts and when the bag is full. Time( full bag) - time( blower starts) it can be hard to keep up with a stop watch, so I use the camera playback to see when each event takes place.