(John McDonnell/The Washington Post)

After tonight’s game in Boston, the Capitals will have played five of eight preseason games, meaning they’re officially more than halfway through training camp.

So technically Washington, with its top two lines all but settled and its bottom two anything but, is right on schedule when it comes to whittling four lines out of a logjam of 18 forwards remaining on the training camp roster.

The opening night top line is chiseled in stone as Marcus Johansson, Nicklas Backstrom, and Alex Ovechkin. The second line’s almost in ink: the acquisition of Mikhail Grabovski answered the second-line center question, and though they’ve yet to play together, Adam Oates seems confident that Grabovski will fit well flanked by Brooks Laich and Troy Brouwer. That line will make its preseason debut tonight.

That leaves 12 forwards in camp competing for six spots. The Capitals are expected to trim their roster in the next day or so, and when they do, Dane Byers, Michael Latta, Garrett Mitchell, and Joel Rechlicz will likely be Hershey bound.

There are eight legitimate contenders for starting lineup spots for the season opener Oct. 1: Jay Beagle, Jason Chimera, Martin Erat, Eric Fehr, Mathieu Perreault, Aaron Volpatti, Joel Ward, and Tom Wilson.

Math tells us that, with eight choices for six different spots, Adam Oates has 20,160 potential combinations at his disposal for those two lines if he ignores natural positions and players’ strong sides. Obviously he won’t, but the possibilities and the pros and cons of each do seem endlessly complicated. The opening night lines Oates posts in seven days will clear all this up, but that’s far too long to wait for his answers to the few key questions that will end up determining his combinations. So let’s discuss those questions now:

Will the Eric Fehr experiment be a success?

Oates decided to try shifting Eric Fehr from right wing to center a line alongside 6-3 Jason Chimera and 6-1 Joel Ward. That sizable trio could provide a formidable checking line, as long as Fehr can handle the extra skating and creative responsibilities centers shoulder.

“It’s a different position for sure, so it’s going to take adjusting,” Ward said of Fehr’s transition. “Whether he’s down low letting us know it where he is, and vice versa, we’re letting him know where we are on the ice. I think communicating is a big part of it, and hopefully it will translate into some positive results.”

Oates saw positive results Friday.

“He’s a guy we used last year in so many different situations, he was very versatile. He played good” Friday.

Fehr said the transition to center’s been “smoother than I thought it would be,” but that he’s still making adjustments.

“I’m just not reacting as fast as I’d like to.,” he said. “You see plays, and instead of just jumping you stop and think, ‘Is that my guy?’ ‘Do I need to be there?’ — that kind of stuff. Hopefully in a couple games it’ll get easier and will feel normal.”

If it doesn’t become normal, Fehr will likely slide back to the wing with no harm done. Should he stick at center it’s uncertain whether he would fit as the third- or fourth-line center, which would leave another question for Oates to answer.

Who will be the other center in the lineup, and what of the guy who isn’t?

If Fehr sticks at center, natural centers Jay Beagle and Mathieu Perreault will be left fighting for the remaining pivot job, and potentially a regular spot in the lineup.

Beagle is the team’s only right-handed center, a distinction that would likely make Caps decision-makers think twice before sitting him on a regular basis, and has become a stalwart penalty killer. But Perreault, who played well down the stretch and in the postseason on a line with Chimera and Ward, has more offensive upside.

He’s also played in only one of four preseason games so far, though he’s slated to play tonight in Boston. Perreault says he doesn’t read much into preseason playing time.

“Marty [Erat] and I have been [in a group] together the whole camp, we really haven’t played. It’s only been what, like four games out here? We’ve only played one?” Perreault said. “I feel like we’re probably going to play the rest of them. They’re just trying to get everybody in, the young guys, give a good look to the young guys, I’m not too worried about it.”

Which brings up another question…

What to do with Martin Erat ?

Is there a place for Erat as third line left wing? Even if Fehr find success as a center?

You have to think there will be, as the idea of playing Erat in a limited role would be a tough sell after Washington shipped out top prospect Filip Forsberg to add the winger at the trade deadline last season.

But Erat and Fehr have never played on a line together. If the Manitoba native sticks at center, whether Oates opts to keep him with his preseason linemates will impact where Erat falls.

Wait, what about Aaron Volpatti?

The left wing’s production potential likely isn’t enough to bump Erat, Chimera, or others from Washington’s lineup, but a move to Hershey seems unlikely. Volpatti could find himself on the outside looking in regardless of what happens with Fehr’s line and its trickle-down effect, but he, too, will be in the complicated mix.