One additional note, lest I create a false impression.  The ratio of spire
height to overall length as a species characteristic, to help differentiate
one species from another, may be useful, when expressed as a species-wide
normal average range (which is really the only way a species characteristic
can be mathematically expressed), provided the mathematical distributions of
the two species do not overlap substantially.  However, it is probably not
as useful as the spire to body whorl ratio, when used to look at variation
within a species or within a population, for the reasons mentioned in my
previous post.
Ciao
Paul M.