Be aware that OFFSET is a volatile function.
The solution on post #4 is not

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Arvi's formula works fine.
If it was returning a static result, then your Calculation mode must have been set to manual.

Yes, that is a volatile function, however the non-volatile Index formula will return the wrong result unless you add 1 to the value in your box D12

As the formula will be obviously in single cell, it will be no problem at all!

I checked sums for different possibilities and made a comparison with OP's sum. As result I decided, that B12 is number of start column in datarange (L4:Q7), and D12 is number of columns to sum.

Btw, the formula using OFFSET allows to use 4 parameters to select the sum range. OP can determine the row from where summarizing begins (currently fixed at 4), and the number of rows to summarize (currently fixed at 1).