Honestly, I don't think it need be that complex. To understand issues, a voter must be able to grasp main points--many simple tests exist for this--and have a lower-elementary school math mastery:
establish whether the potential voter grasps larger versus smaller, has the ability to read "big" numbers, can derive general truths/proportions from from a simple pie chart, and can demonstrate an understanding of at least decimal values such as
Why do you want to test for math when it is such a minor aspect of picking a candidate? Do you need math to know that you don't want to vote for a candidate who said people with your sexual orientations should be sent to reeducation camps? Does elementary math help you decide whether you agree with a candidate's stance on legalizing pot (answer: no, even if you were to read the scientific papers on the subject, elementary math would fall far short for verifying them). Where does math help you when candidates just cherry-pick the statistics that support their point? It does not.
What you really need is fact cross-checking skills and critical thinking. It would also be best if everyone kept the common good in mind when deciding whom to vote for, and had a modicum of empathy (so they ask themselves what a given proposal will do to the people it targets). But good luck testing for that.