You could make money doing something you love (or like [or maybe not so much but just really good at]). It's great to see a spreadsheet come together, and usually a lot better than what you originally envisioned. Especially when the dang thing works.
Protection of formulae : How often have you forgotten to protect the cell contains the formula that took you hours to test and perfect. It takes one instance of honest human error, and that formula is gone.
You may want to create a workbook with multiple sheets for various aspects of your budget. You can have one sheet that keeps track of savings and investments and another sheet that lists your monthly bills.
When your spreadsheet start to hit its limits, what options are open to you? Perhaps the best option is to consider the conversion of your spreadsheet to a database solution. Relational databases became viable not too long after the spreadsheet was well-entrenched as the affordable and quick means of gathering data and performing calculations.
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