tree house build and lag screw strength

Not sure it applies, but with climbing equipment is usually rated for a working load of 10:1. If my 30Kn carabiner can hold 6750lbs, it means the working load is 675. Not sure how much you plan on putting up there, but keep in mind there is nothing keeping you from drilling all the way through and using a big *** bolt instead. You won't kill the tree since threes get there nutrients from the outer area, not the inner. Just don't "skirt" it and it will be fine. You could also put more than one bolt, lag or otherwise to increase the carrying capacity. Finally, if you really want to get nutz, put knee braces under connection points.