The State Canvassing Board will play the role of decider.

What seemed a far-fetched math geek dream this morning came true tonight: On Day Three of Minnesota’s U.S. Senate recount, the Al Franken and Norm Coleman campaigns again increased the number of challenged ballots by more than 40 percent over the previous day. In fact, both campaigns increased their challenged-ballot total by 48 percent.

Ballots that failed to satisfy Coleman’s crew today numbered 404, for a three-day total of 778. Franken’s forces found 387 ballots questionable, for a grand total (so far) of 747.

That makes a combined total of 1,525 challenged ballots with only 60 percent of ballots recounted statewide — already surpassing the 1,500 mark which Secretary of State Mark Ritchie predicted only yesterday would be the number of challenged ballots from the entire recount that the State Canvassing Board would have to review. It’s a number that represents an arms race of ballot-attrition that both Coleman and Franken’s camp condemned today — even as their campaigns ramped up the rejections.

It’s also a number that already dwarves by a factor of 13 the dwindling gap between Franken and Coleman, which now stands at 115. Unless the margin takes a giant leap over the remainder of the recount, the Canvassing Board looks to be the decider in Minnesota’s senate-election drama.

And the Minnesota Independent — which dared to predict in a headline this morning that the challenged-ballot stack could reach 2,500 — turns out to have sharper number-radar than even fivethirtyeight.com’s Nate Silver, who yesterday guesstimated 1,800.

But not to get too high on our high horse, we note that Silver is now looking closely at the number of challenges per 10,000 votes and finding that the Franken campaign on Day Three challenged 7.5 ballots per 10,000, while Coleman’s figures was 7.2, Both numbers represent a doubling of challenge rates from the first two days, Silver says.

Note: The figures in this post are based on the Secretary of State’s official statistics that were released at 8 p.m. Friday. The Star Tribune has been compiling its own totals which differ from the state’s because they include later and in some cases different information. On Friday the Franken campaign claimed its gap with Coleman stood at fewer than 100 votes, counting election officials’ initial decisions on ballots that were later challenged.

(Minnesota Secretary of State Mark Ritchie predicts 1,500 challenged ballots, and fivethirtyeight.com’s Nate Silver estimates roughly 1,800. But since every campaign challenger at every recount site represents an independent variable, I think my guess is as good as theirs.)

More complicated math, working with the rate of increase rather than the hard-number jump, suggests the combined total could hit 2,518. That’s if each campaign were to continue to become increasingly picky over the next two days — at the same percentage rate of increase they showed from Day One to Day Two.

For Franken, the day-over-day increase so far is 42 percent, which if repeated on Day Three and Day Four of a projected four-day recount, would lead to a total of 1,233 challenges.

For Coleman, who had a 44 percent increase from Day One to Day Two, the grand total would be 1,285 challenged ballots, if he were to keep increasing his challenged ballots by 44 percent each day. Drilling down, that means he would challenge 373 on Day Three. And on Day Four, Coleman’s daily challenged ballot count would be – hold on to your hat, Nate Silver! – 538.