Pegster, you are correct it is probably best to trace your own, but for a howto i think we can genralise a bit. What you could do for a custome size is trace it, use a ruler to make a cross and photocopy onto a transperency.
I did a quick Visio drawing and printed it to a PDF file, see attached. Please review.
By the way the idea is not mine i got it from a document i read on the net on
Weasner LXD tips and tricks site.
http://www.weasner.com/lxd/
On this site there is a tips and tricks 1 document that contains a similar template for Schimdt-Newtonians.
http://www.optcorp.com/pdf/meadepdf/TipsTricks1.pdf
I just borowed the idea for our use. But i belive in giving credit.
Regards