I dont really understand what the fields in your example stands for, but look at the tab 'Visual Cues' in the properties for the table-object.
You can put an expression to compare two different columns against each other.
Hope it helps!
please find attached the modified version of your document. I have added a table and solved your problem.
I have changed this "=if (sum(aantgelev)/3>(voorraad),red(200),green(200))" with "=if (sum(Total aantgelev)/3>(voorraad),red(200),green(200))". In your example you need the keyword Total to consider all the Sales.
I have also added a new table with a dynamical formula so you can select 1 or n months and you will always have the correct average of Sales.
Hope it's useful.