I think that your use of any equation as a symbolic reminder of your standards (e.g., to live with integrity), the means (reason) and your primary goal (your happiness) is an excellent idea. It helps you to stay focused on what is important for you. But I think that you shouldn't take the mathematical nature of the equation beyond that. To discuss positive versus negative numbers would be to imply that we now have such a complete understanding of the nature of happiness that each individual can reduce its pursuit to numbers.... and that isn't so. To go that direction with this, into positive versus negative numbers, or to ask about how many decimal places of accuracy we should use, or how do we handle zero, is to go in the wrong direction on this. The right direction is to focus on the key ingredients of happiness that an equation might symbolize as way of reminding ourselves to stay true to our core values with the understanding that they are what produce our happiness. Purpose. That should be the purpose of the equation. I don't think you ever intended to gather numerical data or to crunch the data for its numerical output... right?
Self-esteem is an automatically generated, background sense of ourselves that comes from how we use our consciousness. We can have high self-esteem even if our chosen goals aren't met (although people who use their consciousness properly will, on average, have more success with their goals). If we imagine that our mental machinery, in all of its complexity, has a set of automatic, full-time circuts that monitor how the machinery is used, and that these circuts or subroutines generate negative signals when the machinery is misused, like running a motor that is low on oil, and generates positive signals when the machinery is run properly, that would be a better anology for self-esteem. Each and every organism that has some degree of conciousness has the capacity as its primary tool of survival. The use of powerful claws or swift feet or cunning inventions are all means of survival that flow from having consiousness - it is the primary. The greater the variation possible in the use of a kind of consciousness, the greater the need for a self-regulating device. For us humans, self-esteem is that device. Some values are so because of nature and out of our control as such, e.g., we need oxygen. Some values we create, e.g., medical devices. Some values are cast in subjective shapes, e.g., the flavor of ice cream we like best on a given day. But goals, the things we will make in our minds, and cast forward into the future as our targets, those and formulation of the actions they will require if we are to achieve them are all product of our consciousness. For self-esteem to properly signal successful usage of the consciousness which is understood as I've been describing it - as the tool whose purpose is to guide us in survival and in flourishing - means measuring its use in the ways Branden described. Are we living consciously? I.e., are we avoiding some aspect of reality by not focusing how we should? If so, we get negative feedback for that. On the other hand, if we focus, even when an aspect of reality is painful to face, then we get positive feedback. And so it goes for all 6 pillars Branden identified.