Perhaps this link could add a bit more info about SUMPRODUCT?

http://www.xldynamic.com/source/xld.SUMPRODUCT.html

As for adding rep for good questions I would say since every forum member have the power to give rep points for whatever reason they wish for should not that be sufficient?

Not too sure about this argument

This would make the solutions-seekers feel more valued, and should promote an improvement in the quality of questions asked
as I feel that the majority of OP's after getting an answer "disappear" from the forum hardly aware that there exists a rep system if I consider the number of reps I get after solving a problem.

As many OP's declare that "I'm not so familiar with Excel but a wish for a function or macro that calculates my spreadsheet, walks the dog and moves the lawn" I'm not sure how "improvement in quality" can be achieved especially as in order to ask a "good" question you need to know about 90 % of the answer.

Alf