How to use a specific formula in every row in a specific column now and in the future in google sheets?

In Google Sheets I have a very simple formula to subtract the value in column A from the value in column B and show the total in column C.

Column C formula:

=(B:B-A:A)*24

I’ve selected column C by clicking on the header and I’d like to it to apply to every cell in column C (maybe except the empty cells or header cells).

I think I have it working but it only appears to work on the first row:

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I don’t want to copy the formula to a range of selected cells (filling). I want all cells in that column to use the specific formula and any new cells that are created.

I hope this makes sense. I’ve seen similar questions that copy the formula this is slightly different so please add comments if you believe there is a similar question.