Calculating the leakage area for a door

That formula doesn't quiet work like that. It is used to estimate flow from a know condition (rated test condition) to a new condition. For example, if the door has a leakage rate of 100 m³/h at 50 Pa (solve for C), then you can use it to work out the xxx m³/h at 5 Pa pressure differential (C remains constant...the same for both conditions). The area A can be lumped into the constant C. The flow exponant n, depends on the type of flow...for cracks around doors you're probably looking at 0.65 to 0.5. If you have the same door, and you want to change the undercut...then you could use the A to find the estimate at the new condition.