Ignore empty column in formula

Hello everyone

I am trying to build a simple board like this for every month in 2023:
image

We need to see the difference in Forecast and actual Sales. That’s why I added the simple formula: {Actual}-{Plan / FC} to calculate them. Problem is, since the column “actual sales” stays empty until we know at the end of every month, it always calculates a negative result for the yearly plan if you filter. How can I add a formula that doesn’t calculate a result for the column “Difference” until we fill out the column “Actual”?

Thanks in advance and have a nice day!