Think in terms of the tabs on your word processor rather than
measuring from margins, etc. Dialogue begins at tab 29 and
cuts off at tab 60. Period. That’s all there is to it.

This information was taken from Cole/Haag’s “The Complete
Guide To Standard Script Formats: Part I: The Screenplay.”
You should purchase this book. It is the industry standard
and covers just everything on which you might a question.

Producers, readers, actors, directors, etc. aren’t so much
concerned about spacing as the screenwriter is. There must
be some semblance of order on the page so that it is easy for
others to read. So, dialogue is placed in the middle of the page
for easy access. No one will take out a tape and measure
distances. They measure with their eyes, like what looks right
and what doesn’t look right. Whatever doesn’t look right can
throw them off and cause the screenwriter to lose credibility,
not only in spacing, but the writing as well.

I hope this is of help to you.

Best Regards,

Donald L. Vasicek
Olympus Films+, LLC
The Zen of Writing (Screenwriting)
http://michaelc.nextmp.net/wordpress
dvasicek@earthlink.net