i would like to weigh in on this as GB and i were discussing this about a year ago when we started preaching this revised formula. now i am not sure where GB came up with the (15) in the formula but the first time i encountered it was when i ran the Ultimate Diet 2.0 back in feb 2012. Lyle MacDonald recommends using the formula (body weight x 15) for figuring out maintenance cals. (14) for slower metabolisms and (16) for faster metabolisms.
the train of thought when calculating TDEE or maintenance cals (as i prefer to think of it) is body fat is a non-contributory tissue where daily caloric requirements are concerned, therefore logic would lead one to the conclusion that (body weight x 15) is not extremely accurate. i have speculated that Lyle MacDonald (the author of the book) assumed that his audience would be no greater than 15% body fat (which is what he specifies as the highest amount of body fat you should possess is 15% to run his diet) and thus concluded that on average the difference would be negligible as it pertains to estimating maintenance cals.
this is where (LBM x 15) comes from as i see it. i would also like to note here (and i think this is important) the reason this new formula was devised is because of the gross error in the activity multiplier (specifically 1.55) in the original "TDEE" sticky. my thought on the matter as we worked our way thru to this new conclusion was that 1.55 was high and that 1.2 (sedentary) was closer to appropriate. when i applied the specific math to myself (being the inquisitive guy that i am

) i concluded that (LBM x 15) for me was the same as (BMR x 1.26) which was MUCH more in line with reality than the generously incorrect (1.55) of which i was having to run a deficit of 900-950 cals on my cutting diet.
we will not even get into the (1.75) multiplier..
i hope this clarifies things and that while probably a bit low (LBM x 15) DOES INDEED account for all activity. it is simply an estimate..
