Formula reference in different field on the same column


Hello, probably this is a newbie question, but I don’t figure out how to solve.

I have a simple table with two columns: month and incremental mileage. In the right column I would calculate the month mileage with a easy formula {month2}-{month2}.
But I don’t find a way.

The solution I’ve found is to traspose column and row, so I can use the formula.

But I can’t use a Form to insert new data and, if I would insert a column with fuel consumption, it becomes hard to handle.

Any suggestions?