10% body weight (weights should be when "empty" to be as accurate as possible - they could poop that much sometimes so the starting weight really MUST be when empty) is a reasonable rule of thumb to where it starts being significant, assuming the snake started at a healthy weight. That's on the lower end of significant so it could be argued for it to be higher, but it's enough to suggest a decent probability of an issue and it's good to have some leeway while the problem is addressed. A healthy ball python who just doesn't feel like eating for months often won't really lose any weight at all. Looking triangular is also a problem, looking round is a good sign. Someone with the experience to have a good idea visually like Deborah won't necessarily be concerned at 10% in various cases, but it's a reasonable place to start that would give a fairly minimal number of false negatives on "must or must not be addressed now" without a ridiculous number of false positives.