We start at the highest pressure and work downward for two reasons. 

1. It helps the fans reach the target pressures faster. The speed control algorithm works faster going from a higher pressure to a lower pressure.
2. It helps you properly select your Range Ring for the test. By starting with the largest possible range (highest flow) to test at your highest pressure you ensure the minimum possible range changes while testing. Ideally, you don't want to change ranges in mid test because it wastes time.