For successful forecasting it is crucial to choose correct similar films but also to set the correct scale factor.

Two films are similar if you expect them to have similar life cycle, same pattern. For example most of the visitors go to the cinema on first week from the release date; afterwards we'll notice a high drop in number of the visitors and short life cycle or slow start but very long life cycle. They should also be appealing to the same type of moviegoers.
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Similar life cycle pattern and same target group does not mean that they necessary have same number of the visitors under the same conditions. This is defined by the Scale Factor. A Scale Factor of 2 means that you expect a new film to do twice better than similar one, of course under exactly same conditions. A Scale Factor of 0.5 means that you expect new film to do only half of what similar did.
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The system will provide a suggestion of the Scale Factor to help you, but the final decision is yours.