This page finds a spacing for the guides on your flyrod.
It is based on Wayne Cattanach's original "Guide"
program, which is a clever yet easy method to an
attractive guide spacing.
An alternative is the
Dale Clemens
formula.
The program requires four inputs,
in a choice of American or metric units:

The number of guides. A rule of thumb is to round the
length of the rod to whole feet (rounding up for six
inches), then add one. This gives the number of snake
and stripper guides, not counting the tiptop.

A guide to index from. For instance if you want guide
number six placed just below the ferrule, you are
indexing from guide six. (Guides are counted from the
tip end down.)

Distance of index guide from tip. In inches or
centimeters.

Amount to increase the spacing. Spacing between
guides increases this much for each successive guide.
Wayne suggests 9/16 of an inch (about 1.4cm). Intuitively,
you may want a smaller increase for slower or
parabolic rods or a larger increase for extrafast rods,
to put more guides where the rod is stressed.
After its done, you may want to adjust the position of the
stripper guide a little to get it a comfortable distance
from the grip. Its a balance between aesthetics
and function, like everything else.