Help with forecasting

Hello,

I'm making a forecast with 3 months rolling (Column F) using the Costs (Column E).
When the month has ended the actual cost is being moved to column F and then it forecasts for the remaining months of the year.

However, I realized this is not working correctly.
In March the actual cost is 0. Hence, 0 should be in cell F6.

But the way the formula is working in Column F is based on the 0. So if 0 then take average. This is working as long as there are costs but this is not always the case.

Is there a way to calculate this differently?


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Thanks!