I just wrote a new calculator to work out best fit and cuts for tiles etc.
Its pretty basic. You enter overall width and unit size and it will tell (and show) you smallest gaps (joints) to fit whole units (add or subtract units to test fit)

You can enter gap size to get cut size of last unit, or cut each end unit equally to fit.
You can also start with a cut and work out spacings to the other end.

Is this any good for tiling?

It lives here
http://www.blocklayer.com/Tiles.aspx

.