If you are importing or exporting with other applications, then a particular “absolute” scale might make a difference. But otherwise, a “BU” can be anything you want it to be. As long as, within your project, they are consistent relative to one another. Get out there with your measuring tape (or laser tape), and actually measure real-world things. When placing objects on the set, do the same. Pretend that it’s a “real shoot.”
The proportions are the most key. I think it’s helpful that Blender now offers two real-world units of measure for your convenience, so that both Europeans and Americans can be happy. 
If you are "using ‘scale,’" you really can’t get it “wrong.”