A 'campaign' IS the name that is given (if you don't choose one) when YOU start doing 'promotions'.
Yes, eBay will happily tell you the 'average percentage' needed when doing a promotion; but of course that number is skewed in their favor and may not be in your best $$ interest. Start small and then search a few days later (not signed in) to see where your items stand.
Also note that Promotions are worthless on 1-of-a-kind old/used stuff since seldom are there "keywords" that someone will input to find an 'orange large 1960's glass vase'.
