I have looked at Cornell University Roper Center analysis of past elections D / R / I splits.
For 2004 it was 37 D 37 R 26 I
2008 39 D 32 R 29 I
2012 38 D 32 R 29 I
To be conservative, let’s assume: 37 D 34 R 29 I
Assume 90 / 10 split for each party. Indies go to Trump by 57 / 43.
Trump wins 51 / 49. But, I believe it will be better than D+3 for Trump.
I agree, it should be narrower this year. According to Gallup’s numbers, it’s narrowed by about 1.5 points. That’s just general affiliation...not likely voters, however. But the trend is in the right direction. Now, even Democrats aren’t happy about their candidate and the enthusiasm gap is significant this year.