I recommend that you stay with 1.0 for line spacing. There is no compelling reason to go above or below this spacing. Beginning readers may not catch it but a Senior Analyst certainly will. Don’t forget, they see several scripts a day. Just a simple comparison makes an adjusted script stand out in a less than positive light. A reader then assumes the screenplay is short and this writer needed to stretch it rather than revisit what was written. It also negates the industry standard timing of 1 minute per page.
A screenplay is an invitation to participate not a novel. Consider the ‘graphic novel’ style. The scribe’s goal is to provide the reader / audience with a compelling reason to turn the page / stay in the seat. The fine line between telling too much and revealing too little is where both the craft and art of screenwriting meld.
All professional screenwriting software manages the spacing correctly and in all but very special case the default is the format to use. Also, don’t use their proprietary methods of stretching or reducing a script. Messing with margins, line spacing and font size is a sign that the writer is not yet a master of the screenwriting craft.
Similarly, the number of lines in dialogue, action and description should be minimalist. This leads to the 3 vs. 4 vs. 5 lines or sentences discussion. Think “twitter” for action and description and “haiku” for dialogue and you’re well on the way to writing a sharp and snappy screenplay. Action and description must be in the present tense with the absolute minimum of adverbs and adjectives. When written properly the line spacing will not seem tight.
Disclosure: My in depth seminars re the first page and the first ten pages cover these topics.