Formula for task completion, multiple field option

Hi there ! I have been struggling a lot for i guess a simple formula …
I have a table with contact informations (name, product of interest)
Then a second table for tasks (see attached)


I want that when a product A is selected, to calculate the task completion (as you can see on the 100% cell) the formula I used the following formula : IF({Product of interest}=“A”,SUM({A-1},{A-2}),"")/2

But, if a customer select product A and B, I want the task completion to be calculated with the checkbox “B-1” and B-2" as well … And same in the case a customer pick product A, B and C, I want it to be calculated with C-1 and C2…

I have been looking with IF formula, Switch formula or even Substitute, but i can’t find a correct solution ;( Any good way to do this ? thanks in advance !

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