I actually have a pretty similar approach. (To be honest, Civilization's economic system is not exactly novel.) Except I combine the daily rate with a running

I actually have a pretty similar approach. (To be honest, Civilization's economic system is not exactly novel.) Except I combine the daily rate with a running treasury (each day starts with the previous day's ending balance). All expenses must be paid out of the treasury, so if I want to purchase something I have to save up for a while. Importantly, the 'treasury' is separate from my actual savings, which are deducted at a constant rate from my daily income. The really fun part comes with estimation: I have several years of data now, so I've been able to develop a pretty complex spreadsheet which predicts my network over time. Every month I compare where I thought I was going to be and where I'm at, adjusting estimates to get more accurate over time. It's gotten to the point where I can pretty reliably predict my net worth to within $100. (One time, I even got it right down to the cent, from 6 months out.)