A simple method of determining the approximate length of tube to use
would be to take the length of the 22nd tube and multiply by 88.  This
assumes an 88-note player, of course.

The logic behind this is that the 22nd tube is halfway between the
longest and the shortest, but will not be the mean of the two as the
arrangement of tube lengths will follow a geometric relationship rather
than an arithmetic one.  The resulting total length will probably be
a bit short as the tubes shorter than the 22nd are progressively less
shorter than those that are longer.

Alternatively, take the mean of the longest and shortest, 17 inches in
John's example, and multiply by 88.  This will probably over-estimate
the length.

Anyway, it's always useful to have a bit of stock left on the shelf for
next time.

Best regards,
Nicholas Simons
GB