I'm into simple, easy-to-understand, metrics. One of my favorites is scoring efficiency. This is a simple measure of dividing points by FGA. 20 points is 20 points, but if it takes you 25 shots to get 20, you're much less efficient than if it takes you 12 shots to get 20 points. Pretty simple. Not without flaws as a metric, but pretty simple.
The NBA average right now is 1.24 points per field goal attempt. Obviously, the more 3s you make, the more FTs you make, help your efficiency. Some guys are more efficient over limited minutes, but the more they play, the less efficient they become - kind of like a utility player in baseball...good in small doses but the more he plays, the more he gets "exposed". But other guys are just gifted scorers.
Right now Tatum has 365 points on 238 field goal attempts. That's a ratio of 1.53 points per field goal attempt. SIGNIFICANTLY better than league average. It's better than LeBron, who has scored 705 points on 467 FGA (1.51 eff). It's better than Harden, who has scored 698 points on 465 FGA (1.50 eff). Better than Steph Curry (606 points on 406 FGA, 1.49 eff). Better than Durant (527 points on 378 FGA, 1.39 eff). Better than the Greek Freak (652 points on 439 FGA, 1.49 eff).
I mean, he's putting up elite efficiency numbers. So yeah, to WBCD's point, he EASILY could be a 20+ point scorer just by taking a few more shots. Incredibly efficient scorer.