CARMEL, IND. — Roberto Castro should be a slave to precision. He is, after all, a Georgia Tech man. A civil engineer, says his degree. A professional golfer, says his W-2 form.

And, yet, he committed some quite sloppy math coming in Friday with a share of the BMW Championship lead. As Castro calculated what has to happen here this weekend in order to advance to the Tour Championship just 15 minutes from his Atlanta home, he arrived at but one answer.

“It’s pretty simple for me,” he said. “I have to basically win the tournament to make it to East Lake. So, there’s not a whole lot of shuffling or points thoughts for me.”

Actually, his chances are a little more diverse than that.

Just to clutter his keen mind with some other FedEx Cup points thoughts:

As the leaderboard read Friday, according to PGA Tour numbers people, Castro could finish as low as a solo fourth here and advance to the season-ending Tour Championship at East Lake in two weeks. But stay tuned. It could change with the next downhill 4-footer.

OK, just winning would make everything a whole lot simpler. A victory by Castro would set in quick-drying mortar one of the better stories coming out of the FedEx Cup playoff chase. Former Milton High and Georgia Tech hotshot golfer emerges from nowhere — he was 53rd in the points standings coming to Indiana — to make the privileged 30-man field back home at the Tour Championship. A determined and detour-filled career is finally rewarded with its first victory in 126 starts. A 31-year-old fellow who qualified for only one major this year (T-66 at the PGA Championship) gets into the playoff finale where the prestige and the payouts are piled high against the East Lake clubhouse door.

Yeah, we could work with that.

Back-to-back 65s at the par-72 Crooked Stick Golf Club has left Castro feeling as happy as one can be halfway through a hard tournament. “I’ve probably had a few better ball-striking 36 holes, but that was good, really good from a shot selection, course strategy and execution standpoint,” he said.

For as modest as his results have been this year – there was a runner-up finish at the Wells Fargo Championship in May – Castro never ruled out the chance to make it to East Lake. “No vacation plans or anything during Tour Championship week. That week has been left for golf for a while,” he said. Having gotten a taste for the Tour Championship once before, in 2013, finishing T-9, Castro dearly wants to go back for seconds.

But what’s that sound? A loud dual carb roar closing fast behind Castro?

It was Dustin Johnson, big-hitting U.S. Open champion straightening out Crooked Stick with two eagles (lipping out on a possible third) and a 63 Friday to tie Castro at 14 under.

To contrast: That’s the 6-foot-4 Johnson, No. 1 on the PGA Tour in driving distance and second in scoring average, closing in on the slight 6-foot redhead, 156th in driving distance and 59th in scoring.

A golf writer put it to Johnson this way: “If you were someone who never won on Tour and had the lead in this tournament and looked and saw you behind him, would you be nervous?”

“Yep,” Johnson said. “I would for sure.”

Nevertheless, for his part, Castro continued to hold to his win-or-go-home-but-not-to-East-Lake mindset. Turns out, he had good reason for his stubbornly faulty math.

“When you play great and get a chance to win, you got to finish it off,” he said. “If you’re shooting for third or fourth, you’re probably not going to win.”