We can solve this without nesting a single IF. The formula you need for E1 is `=IF(C1<>"",C1,IF(D1<>"",B1*D1,0))`. That's it. No third layer, no impossible logic tree. Your instinct to reach for nested IFs is a natural one, most spreadsheet users learn early that IF is the Swiss Army knife of formulas. But this is exactly the kind of complexity that AI-native tools are designed to dissolve. You don't need to become a formula architect to track bills efficiently.
Let's walk through what that formula does. It checks C1 first: if there's a dollar amount there, use it as the increase. If C1 is blank, it moves to D1: if there's a percentage, multiply the current amount (B1) by that percentage. If both are blank, it returns zero. The logic is linear, not nested in the traditional sense, because each IF resolves cleanly before the next one fires. This pattern works for any row where you might have either a fixed dollar increase or a percentage increase, but never both. It also avoids the trap of adding C1 and B1 together, which would double-count your current amount. The formula you had, `IF(C1="",0,C1+B1)`, would give you the total instead of just the increase, which is a common misstep when you're thinking in terms of final results rather than incremental steps.
What this really shows is that spreadsheet logic doesn't have to be a barrier. You're clearly someone who wants to keep skills sharp, and that instinct is valuable. But the best skill you can develop is recognizing when a tool is asking you to do unnecessary work. A nested IF with three or four conditions for what is essentially two possibilities is overengineering. Modern spreadsheet tools, especially those with AI assistance, can parse your intent in plain language. You could type "if C has a number use it, otherwise if D has a percentage multiply B by it, otherwise zero" and get the same result. That shift from "writing formulas" to "describing outcomes" is where productivity gains live.
So here's the concrete takeaway: test that formula on a row where you have a dollar increase, then on a row with a percentage increase, then on a row with nothing. If it works, you're done. And next time you feel the urge to nest IFs, ask yourself whether a simple two-step check will do the job. Spreadsheets are supposed to save you time, not consume it with logic puzzles.