Conversely, it may be too long. No method can accommodate a wrongly sized dérailleur. The Shimano method at least centers the arm range to minimize the effects at either end.
Too long a chain is only a nuisance and keeps you from effectively using the small-small combinations. If you do inadvertently use small-small no harm is done, the chain just hangs loose.
Too short can lead to catastrophic damage so if there has to be a choice, other than perfect, go with too long.
BTW, big-big+2 will give you the proper chain length if you are within the recommended tooth count for your rear derailleur and will allow you to use small-small without the chain going completely slack.