How can I find the right distance for "unders"?

What do you mean by 'Unders'?
If you are talking half slate at eaves under the first full slate to close the gap, the batten fixes directly in front of the batten for first full slate in order as you are probably aware to close the first full row gap between slates.

I have a pdf download ready from a site called www.ribaproductselector.com if you want it send e-mail to robertwhiting76@hotmail.com it might answer your question.

Bob
January 2009