I think it's supposed to be 20 for a factory papered edge and 30 for a cut/open edge
The spec sheet has all sorts of information on screw spacings and overhangs presumably used so estimators can price the job.
That sounds logical, bet it never happens on site.
Plasterboard is often joined on the edge of a stud which is 45mm wide -I suppose you might get 20mm if the screws angled a bit and the join is central
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local