I know this is old but I think the current answers are close but slightly inadequate. I think the formula itself (rather than comments/descriptions) needs to account for rounding somehow, so here is an actual formula (in Google Sheets syntax) that does work, and I feel is human-readable:

    =(ROUNDDOWN(A1/2) - 5)

Where A1 is the ability score (or cell).

You could also use truncation, e.g. `TRUNC(A1/2,0)-5`, casting, e.g. `INT(A1/2)-5`, or the floor function, e.g. `FLOOR(A1/2)-5` to effectively turn the decimals into integers. I feel like `ROUNDDOWN` is a bit more self-explanatory.

Mathematically I think the above formula would be expressed, using floor function notation, as:

$$\lfloor\frac{a}{2}\rfloor - 5$$

Where ɑ is the ability score.