Well, perhaps a bit late but...  I have only rebuilt one stack so far.
When faced with the same problem here is what I did:

I figured that (for the most part) each tube will increase by about the
same amount, one after the other (depending of course on the layout of
the tubing).  So, I took the length of the longest tube, the shortest
tube and a tube in the middle that should be about medium length,
averaged the lengths together and multiplied by the total number of
tubes.  Assuming that the tubes graduate in a mostly linear fashion
this works.  Consider the following:

If we had a hypothetical piano of 5 keys with the tube lengths as
follows,

  key 1 = 18 inches,
  key 2 = 16 inches,
  key 3 = 14 inches,
  key 4 = 12 inches,
  key 5 = 10 inches,

we would need 70 inches of tubing (adding all the lengths together).
Now, if we wanted to estimate that, we could average 3 of the lengths
(longest, shortest and middle) and then multiply that by the total
number of tubes :

  18 + 14 + 10 =  42 / 3 = 14
  14 x 5 = 70

If you increase the number of tubes measured you can refine the estimate
somewhat but the tubes need be measured from an even spread.

Now of course some common sense needs to be injected here.  You need to
look at the tubing layout and see what tubes increase by a set amount
and what ones don't.  Only apply this formula to the tubing that
increases by a set amount.  The rest will need to be measured by hand.

If half of the tubing goes to the left of the spool box and half goes
to the right, then this formula still works, only apply it to one half
of the tubing at a time.  (If our hypothetical piano had 10 keys with
5 tubes on each side of the spool box then we would need 140 inches of
tubing or 70 on each side).  However, if the tubing has an odd layout
then obviously this will not work.

Anyway, when I used this formula it worked out very well, but of course
this is only an estimate.  You should always buy some extra tubing as
this formula doesn't account for bends, routing and mistakes.  How much
extra is only a educated guess and will vary depending on the tubing
layout.

Hope that helps,

Ray Finch