Rafal, the CI sort of assumes that you have a straight line as characteristic curve, while your LOESS approach most likely won't give you a straight line but some spline curve with variable slope over its range. I would therefore suggest a slightly different approach: create a model for what you expect as characteristic curve, e.g. a flat line in the b+f region, a straight line in the linear region and some form of smooth transition between these two straight lines. Once you have a model that covers all characteristic curves you are likely going to encounter, you can do an LMS fit to get the parameters for each set of measurement data. It should then be simple to associate a CI with the parameters you obtain for your model.