I am a little puzzled about what you are trying to do. Some distributions do not allow negative numbers, some do.
But, I have attached a simple curve fitting exercise. The Johnson Family Distribution is an extremely good fit at a p value of .95 (where 1.00 is a perfect fit). The normal distribution has a p value of .19, so it is a terrible model for your data. Therefore, using the model that most closely represents your data, your capability is .95. Not surprising, when you are so close to the upper specification. Remember, the model that most closely represents your data is the one that gives the most reliable conclusions. That is the point of all statistical analyses.