am preparing to use an external catheter for night-time prot

am preparing to use an external catheter for night-time protectionl. when measuring for size, should the penis be erect or flaccid? when putting on the catheter should the penis be erect or flaccid? or does it not matter?

When putting on the catheter the penis should be flaccid. But I always account for erections so I tell men to measure the penis flaccid and go up one size to allow for erections.