Average function with Cross tab reports

Hello Experts !

I have a report in which I have brands on Y axis and Months on a X axis of a crosstab . The last column represents the avg. Brand sales of the months and I have applied the Brands function for this.

Now if using filters, there come a brand which does not have sales , lets say in the month of jan- july but had sales from Aug-Nov , the average gets calculated as Brand Sales/ 4 but it should count the months (in the denominator) from the beginning of the year . i.e from Jan . So, if we are in the month of november the denominator should be 11 .