Incumbent presidents' NPV percentage compared to their approval ratings...

[twenty42]

2012 Obama
Approval rating: 50.0%
NPV percentage: 51.1%
Spread: +1.1%

2004 Bush
Approval rating: 49.1%
NPV percentage: 50.7%
Spread: +1.6%

1996 Clinton
Approval rating: 54.6%
NPV percentage: 49.2%
Spread: -5.4%

1992 Bush
Approval rating: 32.6%
NPV percentage: 37.4%
Spread: +4.8%

1984 Reagan
Approval rating: 57.9%
NPV percentage: 58.8%
Spread: +0.9%

1980 Carter
Approval rating: 37.9%
NPV percentage: 41.0%
Spread: +3.1%

1976 Ford
Approval rating: 43.6%
NPV percentage: 48.0%
Spread: +4.4%

1972 Nixon
Approval rating: 61.3%
NPV percentage: 60.7%
Spread: -0.6%

Averages since 1972
Approval rating: 48.4%
NPV percentage: 49.6%
Spread: +1.2%

So incumbents tend to slightly outperform their approval rating in most cases. Of course, the oddest case here is 1996 Clinton. Apparently, there were quite a few people who liked Clinton but still voted against him. It is possible that Perot threw a wrench into things, but GHWB still managed to outdo his approvals by almost five points in 1992 when Perot did eleven points better. It's also possible that there were quite a few voters who liked more than one candidate in 1996, but preferred Dole or Perot over Clinton. I can't imagine liking more than one candidate today.

I'm not assigning any predictive value to these numbers vis a vis 2020, as the numbers obviously show a wide range of possibilities. But it is worth noting that the average approval rating for an incumbent is 48.4%...not a far cry from Trump's current numbers.

[twenty42]

So given Trump's current approval, that'd predict 43% of the vote. Which almost certainly isn't enough to win unless there's a massive left wing third party presence.

If it is a 1976 or 1992 situation, that could put Trump at 47-48%. If it is a 1996 situation, he could wind up with 38-39%. Like I said, there has been a wide range of outcomes so it's hard to place any predictive value in the average.

Bear in mind too that presidential approval ratings tend to be higher in Year 4 than in Year 2.