If a pt has to lose a lot of weight (>100lbs) what percent of the excess weight should be considered not able to be shedded, (ie. not fat tissue ) ?

A lot more to it: First, measures of fat content (calipers are good, plethysmography might be better, MRI is best but..... a research tool). Second, unless exercise is also done, most of the weight lost will be muscle not fat. Last, 10% of total weight lost often results in good outcomes and this is considered a good result even with surgery even if the person remains morbidly obese. (certainly re-eval at 10% loss)