I would make this a bit longer to make sure it is understood right. You have to take an entire additional series and divided it by 2 before you add it. You can't just take half of a sereies. This is a fine but important distinction. If you simply say 50% or half of a series, somebody might decided just to take every other value in the series or its first half or something like that.

Something like "Addition of the product of an additional series and 0.5." might sound long, but it is necessary to avoid ambiuity.