There's no price at which it "should" open. That doesn't mean it won't open at 39.45, though.
"That would be 1/2 of the after hours high"
Has nothing to do with where it will open. If it does open at 39.45, it won't be because that's half the after hours closing price today. It will open at whatever price someone is willing to pay. One half of today's regular hours closing price will be the basis upon which Monday's gain or loss will be calculated. That's the only thing you can be sure of. ... well, that and death & taxes, of course.