Three small habits keep a model from showing errors or drifting by a cent.
Test before you divide. A margin, a growth rate or a ratio fails with #DIV/0! when the bottom number is 0: a product not launched yet, a region that opened this year, a company with no debt. Test for it: =IF(B8=0,"n/a",C8/B8). "n/a" says honestly that the ratio doesn't exist; 0% would claim a margin that isn't there.
Round where the money is rounded. Tax is charged per invoice line and rounded to the cent, so model it the same way: =ROUND(D6*$B$3,2) on each line, then add. Rounding the total once can differ by a cent or two, and that's the kind of gap that makes a check fail.
Use IFERROR sparingly. =IFERROR(C8/B8,"n/a") is shorter, but it hides every error, including a typo or a broken reference. An IF on the specific condition only catches the case you expect.