Budget versus actual formula

Have a spreadsheet with column headers for each month and row headers of budget items (such as electricity expense) as well as a column for the monthly budget figures for each expense item. Each month I enter the individual expense item figures and it automatically sums the row of figures and puts it in the cell to the right in the column labelled "Actual Year-to-Date".
My problem is what formula do I put in the cells with the column marked "Budget Year-to-Date" because each month it would be different. Right now I use a formula that multiplies the cell that contains the monthly budgeted amount for the item times the number of months that have gone by, so each month I need to remember to change the number of months and copy the formula down the column. For example instead of B3*5 for May, in June, I will need to change the formula to B3*6. There must be a better way to do this!