You might be better off figuring out where your ads will go, how often they'll go there, how big they'll be, and what those outlets charge for such an ad. Because although an approximate percentage works for business plan purposes, in reality, what your local newspapers etc charge for an ad will dictate things much more than your weekly ticket sales will.
Ads don't get cheaper just because nobody turned up last week, you know?
But FWIW, I used 4% in my biz plan. Always better to overestimate and be pleasantly surprised.
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