W4RNL has a number of articles on quads, along with some
design equations implemented as a BASIC program. The wire
diameter is a critical variable - fatter wires require a
large element diameter. Any formula that doesn't take
this into account is suspect.
You can see some of his research here:http://www.cebik.com/quad/quadlist.html
I've built 2-, 3- and 4-element quads from his formulas
and they have all come up right on frequency without
any tuning or adjustment.
There was an article in the first issue of the ARRL Antenna
Compendium that had design charts for quads (that also
included the effect of the wire diameter.) One of the
interesting outcomes was that quads with an odd number of
directors would have wider bandwidth and were more forgiving
to tune than those with an even number of directors.
I don't know of any software that implements his charts
and formulas, however.