I calculate my FIRE net worth based on two things, really: The projected assets I will have less the projected liabilities I will have. Roughly speaking, I expect my liabilities to drop by about 1/3, and I expect my assets to roughly double. The combination of those two factors result in an approximately 11-fold increase in my FIRE net worth.
My liabilities are dropping, in turn, because every month I pay down my mortgage, my student loan, and reduce my number of remaining child support payments by one.
My assets are going up, in turn, because every month I contribute roughly 1/3 of my gross salary into savings, and my existing assets I have scheduled to grow based on historical rates.
Without getting into specific numbers, I don't really require very much to live on, and each month the vast majority (about 70% if I calculated that correctly) of my reasonably good income goes towards the above mentioned items.
HTH,
2Cor521