On Sat, 27 Jul 2013 12:57:02 +0100, Dave W
wrote:

I assume you mean 15 degrees below horizontal.
"Depenfing on the width" - of what, measured how?
I assume you measure the extension E down the sloping face, not
horizontally.

X=Y+Esin15
i.e. X=Y+(0.26 x E)
Your example would be X=2.1 + 0.26x2, i.e. 2.62m

If you can add the Y in your head, all you need to remember is that X
is 0.26xE above Y.

Even easier, as sin15 is pretty much 1/4 (and how accurate is your 15
anyway), X is E/4 higher than Y.
--
Dave W