oops, sorry for mistake
Too bad there is no rule to decide if a single layer disc can be used safely or if is better to use a DL disc.
The amount of compression that can be applied safely is related to the source movie. Some movies can be compressed also at 70% with no big quality loss, and other can give ugly results also if compressed at 90% only.
The only way to decide is to run a test for each movie. Compress it to a single layer disc, burn it on a rewritable disc and watch result on your television. If results are good, then a single layer disc can be used safely.
It is a time consuming procedure, but it is also the best way to decide which disc is better for that specific movie