Is it harmful to do a lower dose of prednisone than prescribed for my asthma, even if the lower dose seems to be working fine?

Talk with your doc: It is quite possible that a lower dose of prednisone than prescribed is all you need to treat your asthma, but though you feel fine at the time of the lower dose it may not have been enough to resolve the inflammatory process that caused the attack. That can result in a rebound attack more harmful than the original one. Tell your doc what you've done, they will guide you on further care