The 2DS may have the same (virtual) screen size, but the gap is a world apart.
(Left: a 2DS, Right: an o3DS)
Now, the virtual screen size is certainly a known, but the gap isn't.
If we either had a raw screendump of the 2DS's panel, or simply knew its actual resolution we could subtract 240+240 from the hight (Assuming there are no unused pixels above or below the screen)
Measuring the size of the gap and working back from there is pointless as it won't give anything more accurate than the comparison I've already done, which game 36 pixels, but I wanna know the true value none the less.
I'm sure you can understand, in your guide you did talk about doing things the proper (nintendo) way.