miller orr on excel

Firstly, I assume you are asking for the purpose of the computer based exam. The exam does not use excel (although the spreadsheet is very similar - you can find specimen exams on the ACCA website). As far as calculating it correctly, you type in the same formula in the spreadsheet. If you are having problems with this, then type out your workings, do the calculation on your calculator, and then type the answer into the spreadsheet. Just make sure that you do type out your workings - the marks are for the workings rather than for the final answer, where the workings are typed out or whether they are a formula typed on the spreadsheet.