Write a Google Sheets formula that won't quietly break when three other people are editing the same sheet

You're helping build a formula for a Google Sheet that isn't just used by me — it's a live, shared sheet that a few other people edit directly, which creates failure modes a single-user spreadsheet doesn't have.

CALCULATION NEEDED
Flag any row where actual spend exceeds 110% of the budgeted amount for that line item.

SHEET LAYOUT
Column C = budgeted amount, Column D = actual spend, both starting row 3; row 1-2 are merged header cells.

WHO ELSE EDITS THIS SHEET
Two project managers who update actual spend weekly by typing directly into cells, neither is comfortable with formulas.

HOW IT'S USED DOWNSTREAM
A conditional-formatting rule and a summary pivot on a second tab both reference this flag column.

Build the formula, then check it specifically against these shared-sheet risks:
- Would this formula silently break or shift incorrectly if someone inserts a row in the middle of the range it references?
- Does it rely on any cell reference that a non-technical collaborator might reasonably type over (a hardcoded threshold value sitting in a random cell versus a labeled input cell)?
- If this feeds a chart, pivot table, or another sheet via IMPORTRANGE, will a formula error in one cell (like #REF!) propagate and silently break those downstream views, or fail loudly enough that someone would notice?

If the answer to any of those is "yes, it would break silently," revise the formula or the sheet structure recommendation to close that gap — for example, using whole-column or named-range references instead of fixed cell ranges, or pulling a hardcoded value out into a clearly labeled, protected input cell.

WHAT NOT TO DO
Do not recommend a structural change (like protected ranges or a separate inputs tab) without explaining specifically which risk it closes — every recommendation should map to one of the three risks above, not be generic "best practice" advice.

OUTPUT FORMAT
1. The formula, ready to paste.
2. For each of the three shared-sheet risks: does it apply here, and if so, what was changed to address it.
3. Any sheet-structure recommendation (named ranges, a protected inputs tab, etc.), each tied explicitly to a named risk.
4. One line noting if any risk remains open and why it's acceptable to leave it, if you conclude that.