You have to know a few things first: the density of the air ( Greek letter Roe but here we will use a small "p")at a height above or below sea level, the velocity (V) of the moving air and cross sectional area (A) of the supply duct and each branch or runout. Since PRESSURE(total)= PRESSURE(dynamic) + PRESSURE(static), if you know the total pressure of your HVAC system and can measure the velocity of the air at points in the ductwork, then you can determine the static pressure. Since Pdynamic = 1/2"p" X (V)squared, you can take the total pressure and subtract the dynamic pressure to get you static pressure where you can measure the actual velocity. If you design a constant velocity duct system, the layout of ductwork will produce just that -- a constant velocity throughout the system which means your trunk must reduce proportionally as you split off branches and supply them with air. The moving air will act just like water when you hold your finger over the end of a garden hose. The water has a smaller opening to travel through at the same volume so it moves faster to keep the rate the same for a smaller cross sectional area. If you design an equal friction (or contant pressure) system, you want to maintain a certain static at your branches. You can obtain air flow charts and ductwork calculators from HVAC companies which will help you easily dial to the right airflow you want for each branch and will help you size both the main trunk components and the runouts.