Most spreadsheet problems are not formula problems. They are layout problems. When inputs, workings and results are mixed on one sheet, nobody can tell which cells are assumptions and which are calculated, and a single overwritten formula can quietly change every answer.
The fix is a simple three-part layout. Inputs hold every number you typed in: prices, volumes, rates, dates. Nothing on this sheet is a formula. Calculations hold the workings, and every cell there is a formula that points back to inputs or to other calculations. Outputs hold only the answers someone else needs to read: totals, summaries and charts.
Three habits make this layout work. First, give input cells one consistent fill color so they are visible at a glance. Second, never type a number into a formula; if a rate of 8% matters, it lives on the inputs sheet and the formula points to it. Third, keep calculations flowing in one direction, top to bottom and left to right, so a reader can follow the logic.
Try this: open a spreadsheet you use now. Find every hard-coded number inside a formula, move each one to an inputs area, and point the formula at it. Then change one input and confirm the output moves as you expect.