Calculation to determine total based off 2 fields

[JD2775]

Hi,

I have a tbl_Grid set up in my database. It displays as a portal based off the Plate_ID number chosen, with each ID number being the unique identifier in the portal. There are 10 records per Plate_ID so the portal is 2x5. I need to add a field "Count" to my table. What I need this field to do is total the number of "x"'s imported from Excel based off the SIID field. In the example below the count for SIID 111111 would be '8'. That number would appear in every portal square associated with SIID 111111. At the same time, all "x" squares would turn red (that part is easy).

I need to know the calculation to get the count though.

Thanks

*EDIT* I tried to attach an Excel file example unsuccessfully, and copy and paste some Excel data as an example which ended up all jumbled on the screen. So unfortunately I am not sure how to show an example of what I am talking about to make this less confusing.