Page 38: Corruption
Marks of Corruption is rolled when you go above your corruption treshold. As long as you keep rolling more than the target, you can go pretty high beyond the treshold without any ill effects, and even if you roll under it, you don't get any permanent effects, unless on a roll of 1 or 2.
I think that the higher numbers need some beefing up! You can only get 20 as a result if your corruption is at least 20 points above your treshold. That should always be very serious, no matter how well you rolled. I'd say that there should be very real danger of losing your character on results 15 and up.
I'd turn the table upside down, where results 1-5 are mildish (mostly hideable appearance changes), 6-10 are more serious (taste for raw meat and cadavers etc.), 11-15 are really bad (one point of permanent corruption + very visible marks, like black eyes, split tongue, tentacles), 16-19 are horrible (two points of permanent corruption + random acts of terror and very public displays of corruption), with 20 you turn into a blight born! This way you would gain more and more marks and errational behaviour, if you don't stop in time and you would be put down before you have a chance of turning into abomination.
Of course any number you rolled didn't reduce your current corruption, but you need to rest and do some cleansing rituals to get rid of it.