I have a sales log that tracks vehicle type (cars, trucks) and inventory type (new,used). The data sheet is a straight forward row and column deal with pertinent sales data for each sale event. As part of a daily reporting protocol the sales log information is copied to a report format that breaks out the inventory by type and vehicle. The form shows the number of each type sold for the day as well as the MTD count for each as well. I am using a index function with nested match functions to find the information for the form. My issue is that I am trying to dynamically increment the MTD unit number at data entry time. My desire is to prevent the user from entering the wrong info by having the sheet "calculate" the proper MTD unit count. I have considered a VBA macro but would like to avoid this if possible. Any suggestions on a function driven solution?

This is the index function on the report page that finds appropriate sales data:
INDEX(SalesLogDB,MATCH(RprtMnth&$A$3&"New"&$B5,RprtKeyRng,0),MATCH(C$3,SalesLogDataHdr,0)

In this $A$3 holds the vehicle type data, $B5 is the MTD unit number to search for, and C$3 is the column name to search for. By combing the sales month with the vehicle type and inventory type I create a "KEY" for the sale. So if it is July and we sold a new car and it was the 53rd car of the month the key would be "JulyNewCars53". My issue is generating the 53 in this case as the user is entering the data.