@burcinc-- If you don't have an idea for an expense at all, then of course, you have to do some calculation. Do you use online banking? The easiest way to calculate expenses is to look at your bank account summary or the transactions on your online banking account.
Just add up the payments for each expense. For example, add up payments made at all grocery stores and restaurants for one month. You could calculate for two or more months and take an average for a more accurate calculation. Then use this on the spreadsheet like the article described.