Calculating Drag Increase with Airbrake: Semi Span

[Pulp]

I need to calculate the increase in drag due to an airbrake, however the formula includes "semi span". I'm unsure if this is simply wing span/2 or is it the length of a wing in which case it would be (wing span - fuse width)/2. Any help would be good.

 

[Ryoko]

Semi span is half the wing span.