error in index+match formula

Hello

i have a error in my formula
when i have a blank cell (D3 in example)
it should report the (last value-1) writed in line 3

please see example

Perhaps:

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for B7 & B8 simply add a result vector ($B$2:$E$2) to the above

"it does not work" is helpful a post as you can imagine.

The formula does work so I suspect the issue is down either to delimiters, function translation and/or other inaccuracies.