|Obviously without your data, it's hard to test your formula.|
That said, just from reading your post, maybe I'm missing something but consider this...
The syntax of the INDEX function is:
=INDEX(array, row_num, [column_num])
Breaking down your formula, I get:
In your post you said:
"Match AS7 comes up with 241+2 = 243 Column IV"
"Match AQ10 comes up with 1 Row 8"
While I'm not quite sure what you mean by "1 Row 8", it looks to me like perhaps your matches are reversed. You don't have 243 Rows in your array, so Excel is telling you that you are referencing a cell that is outside of your array.
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