Custom Grand Total

Dear Tableau Experts,

I need help please in doing the custom grand total in my report. I can get the grand total for all the three categories, Model A - C but I want the grand total to compute only for Model A and Model B, excluding Model C. I don't want to remove model C from the view, I only want it to be excluded in the Grand Total computation. The grand total needs to look like the same pie chart also.

In the end,  the no of M in the Advanced Level should be 2 and not 4, the no of F should be 3 instead of 5.

Can someone tell me please how to achieve this?

Thanks so very much in advance for the help.

Regards,

Juhn