We have no idea how the NHL settled upon 27 games as the length of the suspension imposed on Austin Watson after the Nashville forward pleaded no contest to a domestic assault charge in July. The league has no domestic violence policy. Matters of this sort are left to the commissioner’s sole discretion under Article 18 of the CBA.

Similarly, we have no idea why Shyam Das, the arbitrator who heard the grievance filed by the NHLPA on behalf of its dues-paying member, reduced that suspension by a full one-third, to 18 games. There was no precedent to examine, other than the NHL’s ongoing indefinite suspension of Slava Voynov, disciplined in 2014 following his arrest for domestic violence.

The league has expressed its deep disappointment with the decision. The PA issued a statement in which the union essentially explained its responsibility to the appeal process outlined in the CBA while proclaiming its commitment to educating its members on the domestic violence issue. Citing sensitivity and privacy, the union has not made the arbitrator’s decision available to players or agents.

So we don’t know. Don’t know why 27 and sure don’t know why it’s now 18, though we do know that arbitrator Das is the same fellow whose vote overturned Ryan Braun’s 50-game suspension for violating MLB’s PED policy in 2012. The arbitrator, who has also worked on NFL cases, was soon fired by MLB. He is likely to face that fate again.

We don’t even know why Watson has been allowed to practice with the Predators throughout his suspension while Voynov — who wants to return to the NHL but hasn’t found a willing partner after three seasons in the KHL — was barred from the rink before he self-deported to Russia in 2015 after spending 45 days in jail as a consequence of his plea of no contest.

“Different facts and circumstances,” deputy commissioner Bill Daly wrote in an email to The Post. “I can’t really be more specific than that.”

Further complicating matters was a statement released by Watson’s girlfriend, Jean Guardino.

“My behavior and state of intoxication lead to the police being involved that day,” Guardino said. “I have struggled with alcoholism for many years and I am actively involved in AA. I am fortunate to have Austin’s continued support with my treatment. We handled matters poorly on June 16th and know that we need to make better decisions going forward.”

Her statement comes three months after Watson pleaded no contest.

So we don’t know. We don’t know whether to be outraged by the initial sentence imposed following an altercation between Watson and his girlfriend that prompted Class A misdemeanor charges. We don’t know whether it was too lenient, too harsh, or right on the money for this type offense. We don’t know why in the world Das found the penalty excessive. We do know the NHL/NHLPA needs a domestic violence policy, but that hardly will clear up matters. Chances are that Gary Bettman will be loath to relinquish his collectively-bargained power that includes the option of terminating the contract of a player judged guilty of conduct detrimental to the league.

Here is something else we do know: The Nashville organization disgraced itself by allowing Watson to participate in the festivities preceding their Oct. 9 home opener. Tone-deaf while absorbed in self-aggrandizement following a Presidents’ Trophy season in which the team lost in the second round of the playoffs, Watson was introduced in full uniform with the rest of his teammates. Of course he was cheered.

Perhaps ownership and management will cite the terms of the suspension as an excuse for including Watson, Nashville’ 18th-overall selection in 2010 who has 47 points (23 goals, 24 assists) in 216 career games, in the ceremonies that laughably included the raising of three banners. But there is no good reason for this appalling lapse in judgment. No good reason at all.

That we know.

Meanwhile, if it’s $8 million a year for William Nylander, we’ll pass, too. The question is whether the Leafs would force Michael’s son to sit out the NHL season if this impasse is not settled by the Dec. 1 signing deadline or whether they’d be willing to wash the headache out of their hair by moving him for a top four defenseman.

You look at the league’s top first-lines and they’re all anchored by players selected with high first-round picks. Except the one that consistently might be the NHL’s best. That’s the one in Boston featuring Patrice Bergeron, 45th overall in 2003, between Brad Marchand, 71st overall in 2006, and David Pastrnak, 25th overall in 2014. So it can be done.

By the way? Zdeno Chara, signed by the B’s in 2006, is the only free agent signee of the cap era to win a major postseason award, the ageless wonder capturing the Norris for 2008-09.

The NHL has become the on-ice version of Country of No Old Men, but there most certainly should be room in the league for Dom Moore, the 38-year-old unsigned free agent center.

Moore, who was inducted into the Harvard Varsity Club Hall of Fame on Saturday, is skating with the Harvard varsity as he awaits an offer. A fourth-line center who can play up, the 12-year NHL veteran has the skill set that features the ability to elevate his game in the playoffs. Can be nasty, too.

That stamps his value to contenders that may be experimenting with kids now but will turn to vets later.

So who do you have as the Rangers’ all-time No. 18, Marc Staal or Walter Tkaczuk?

The Slap Shots Top Five Blueshirt 18’s: 1. Tkaczuk; 2. Staal; 3. Lynn Patrick; 4. Mike York; 5. Tony Granato. Honorable mention: Mike Ridley. Dishonorable mention: Zdeno Ciger. Also dressing: Trevor Fahey (for his one NHL game that, honest, I attended at the old Garden on Jan. 10, 1965).

Finally, that Peter DeBoer offside challenge of a Rangers goal at the Garden on Oct. 11 that created a delay of 6:58 between the time the puck crossed the goal line and the following faceoff? Angel Hernandez couldn’t make up his mind.