Shooting has 3 basic indicators: 3p%, 3PA and FT% (FTA can only be used as an indicator of FT% reliability).
Having 3p% under .350, 3PA under 1.5 per game or FT% under .700 makes you a suspect shooter. 3p% over .400 and 3PA over 5.0 per 36 minutes and FT% over .800 (maybe in adjacent seasons) allow to consider one a very good shooter.
There's also a matter of two very different skills: pullup jumper (coming to a complete stop and then making a balanced jumpshot) and set jumper, when good shooters usually only turn their torso towards the basket upon catching the ball, since their feet are usually set (sometimes ball comes from under the basket, so there's no movement of the body other than jumping straight up), and balance between the two often skews resulting percentages. Plus some guys surprisingly excel at first, but not the second.
Jason Kidd has shown to be decent in college already in all 3 markers. He just drifted towards more set jumpers as he aged, so his improvement actually wasn't that impressive.
Eurocup teams are rather poor, and two thirds of them are worse, than the very bottom of ACB. And in ACB Porzingis showed to be a 31 3pt% (4 shots per 36, almost all of them spot shooting) and 75FT% shooter, which makes him a solid-to-good set shooter.
There is a lot more to shooting than just percentages, I will certainly agree with you on that, but for time's sake, % is quick and easy. I never played college ball, but I did play in high school and the only thing I was ever good at was shooting, and I used to be really good. I naturally gravitate towards liking shooters more than any other types of players, so I critique the hell out of them. For a 7-1 individual, Porzingis has solid form. Again, we don't need him to be a 7-1 version of Ray Allen, but even if he is at ~42% to 45% from mid-range, he would be enough of a threat that his defender couldn't cheat off of him. I will say this, I am more confident in Porzingis's ability to become a good shooter (for a big man) than I am of WCS for the reason you already said, the style of play in Europe. In regards to Kidd, he definitely improved drastically and now he is in the top 6-8 all time in threes made. Go figure. Ibaka is certainly an exception, but perhaps it was something he was decent in to begin with and never allowed to really use it.
The thought of a 7-1 big man who can shoot the mid-range at a respectable clip, be a weak side shot blocker, and be able to drive hard to the hole definitely deserves heavy consideration.
1. Weak side shot blockers come with different types of impact. And I already posted numbers, that show Porzingis was pretty far off from elite.
2. Porzingis' in-game drives are rather slow straight-line ones - he can't use those in NBA regularly.
Then there's a matter of his relative defensive ability at PF with his frame - he will be a plus defender, but how big that plus would become?