What do you mean by safe CO?

The AFR varies depending upon rpm and engine load, so where as you might expect 1 - 3% CO at idle and light cruise conditions, heavy load may see 4 - 5% CO right up to 7-8% CO at high boost.

If you're asking is 5% ok at idle, well its outside the legal limit for MOT purposes but it wont cause your engine any issues. I'd be more concerned what the fueling is at normal running conditions than at idle so get it set up and checked over.