Jamie, Kowechobe and others
Steve's focus on mass is actually pretty simple. Most everyone already does it intuitively but Steve has developed a formula to give more predictable and consistent results. He's removed a lot of the guess work.
The idea is to design and tiller the bow such that there is just the right amount of mass for the intended draw weight. If the limb is not wide or long enough then the bow will be "underbuilt" and start to take excessive set. If the limbs are too wide or long then the bow is overbuilt thus wasting energy moving the limbs instead of the arrow. Balance. It's actually nothing new per say just a refined and quantified way of applying a principle we have already learned.
I'm oversimplifying of course and if I'm off base then I'm sure Steve will come along and explain it better.
Steve, I know you've posted this several times but if you don't mind could you post your formula here again and then apply it to your bow that you've posted on this thread as an example. Walk us through it. How you not only use the formula but how you approach tillering the bow to reach or meet the mass goal that the formula predicts.