Use ARRAYFORMULA to expand results

I need the column of results to grow automatically when we add rows. Put the calculation in the header row so new entries below get the formula without me copying it, and verify it tolerates blank rows and returns blank for empty inputs. Use ARRAYFORMULA to expand the results down the column and leave the header cell as the written label. Check the sheet where transactions are entered and the header is 'Calculated Margin' in column F, with raw numbers in C and D.

Before I hand this sheet to the marketing analyst, make the expanded formula readable and robust. Surface the single-cell ARRAYFORMULA at the top, keep the header text 'Calculated Margin' visible, handle blanks and text gracefully, and comment the logic so a non-formula person can tell where the inputs come from. Flag any rows that will break the expansion (like stray text in numeric columns).

I put an ARRAYFORMULA into the 'Calculated Margin' header in column F referencing amounts in C and D, but after pasting a few new transactions the results didn't fill for some rows. Priya in ops will start using the sheet Monday and I can't have missing values. Tell me the likely cause and the best fix so the expansion is reliable given some blank rows and occasional 'TBD' text in C or D.

Over months, imports and collaborators keep wiping the per-row formula so I spend time copying it back. I need a practice change so the expanded calculation survives imports and casual edits. Recommend one habit change plus a structural fix to the workbook that stops this recurring work and keeps the header-based ARRAYFORMULA authoritative for column F.