Yeah, I just mean the angle DAE is the inverse tan of 1/20, in the case that DE = 5cm (a common real example of what it might measure in my situation), of course you don't need to include the fact that XB = 8cm, this just shows tan-1 of DAE is 8/160 rather than 5/100, i.e. the same thing. Of course specifying DE = 5cm is not a generic example!