How long should I cut my pipes?

[thomasr]

Could someone tell me how long to cut a one-inch pipe to get middle C?

I am trying to make a set of pipes to use as primitive brass instruments
for a chidren's music class.  I understand that I should operate on the
following formula:

       x is do
7/8 of x is re
4/5 of x is mi
2/3 of x is so
3/5 of x is la

But I don't know any formula for setting do.
Could anyone give me such a formula, or tell me where to find a chart?