Estimate/average out over reporting issue

Hello all,

This is challenge has been driving me crazy today & I'm not sure what I'm after can be achieved.

I have a monthly volume of numbers for the year starting October - September, I know that October - June are good volumes. However, we encountered an issue during months July, Aug and Sept and the volumes have been over reported, by how much I have no idea.

What I need to do is estimate/average out the three problem months to a realistic volume.

I thought of averaging out the good months and with this figure I could get the % increase of three issue months then somehow remove the % uplift as a volume.

However I stuck at the theory...

I may be making no sense at all and an average is the closet I can get... But please another set of eyes/mind would be appreciated

I would suggest that if there is no other way to recover the data for the last three months that your approach is correct. However, that's based on the proviso that the data is not variable by environmental or seasonal factors, for example temperature if you were measuring energy consumption. If it is, I would use an average of the closest relating data of the year, or of historical data over previous years at the same point.