Unfortunately, Danbooru's rating system is quite subjective. Kind of ironic considering that this site thrives on objective tagging, but something has to act as a filter for people such as myself who don't want to look at porn all the time.
Whenever I tag an image as safe versus questionable (I rarely ever upload explicit images), I always ask myself, "Would it be appropriate if my young nieces saw this image?" since at least one of them is starting to like the anime art style. It sort of acts as my radar for what is considered safe to show, and then I follow my judgment from there. Since I hardly post anything beyond revealing skin and sexually suggestive images, I don't know how much help I can be here.
Showing off of a little too much skin, even if it's full on nudity, is a far cry from explicit though. I generally tend to think of explicit images as things that are done to characters such as bondage, torture, sex and violence, which a child would not really understand and we would not feel comfortable explaining (plus we might get in trouble with our country's laws). So questionable would be a step up from that such as bodily functions (peeing, vomiting, bleeding, etc.), some revealed skin, flashing/mooning, sexually suggestive images, light bondage, basically anything that wouldn't incite a huge reaction other than a few raised eyebrows. Then characters who are properly clothed or censor the nude bits cleverly and are doing perfectly normal everyday activities would fall under safe.
Again, this is not a definitive method for rating images. It's just what I use, and I've found it fairly reliable.
