ADB, the mass theory was more based on hunting weight bows, I have found on circular tillered bows it starts to wander off to the high side after about 70#, also as discussed in the book certain woods will tend to come in lighter, yew and cherry for instance. Your bow comes in at around 829 grams not allowing for yew wood if it is circular tillered. If I were to rewrite that chapter for heavier bows it would come in about 10% lighter. Tillering the bow for low set as you achieved will always overide predicted mass which is more of an estimated mass to arrive at close dementions anyway.