-I'd expect (barring injury) Lewis to be a RB1 in PPR, and a RB2 in non-ppr at the end of the season. I'm expecting him to get about 75-80 receptions, maybe 100 carries, and finish with 6-8 TDs. That could sneak him into the top-5 in PPR, but it's not "a lock."
OK, this is useful info.
In 6 games, Lewis has 45/220/2 rushing and 35/349/2 receiving on 45 targets. However, I know you like to discount game 1, since Blount was inactive. In the 5 games Blount was active, Lewis has 30/151/2 rushing and 28/298/2 receiving on 40 targets.
In one of my PPR leagues where I can easily sort for weeks 2 through 8, Lewis ranks #6 in ppg over that period, at 19.38 ppg. However, that ignores that he played at less than 100% in his only outlier game. Consider his scores for the season in that league: 16, 25.8, 17.7, bye, 23.3, 6.9, inj, 23.2. The 6.9 point game is an obvious outlier, and we know why. But even including that game and ignoring his week 1 game, he is #6. Two of the guys above him (Foster, Charles) are out for the season, meaning he is performing as a top 5 RB right now.
In my only non-PPR league, we count 0.5 points per first down rushing/receiving, so it will be a bit different than typical non-PPR leagues. Sorting for weeks 2 through 8, Lewis ranks #10 in ppg over that period, at 15.78 ppg. His scores for the season in that league: 14.5, 21.3, 14.2, bye, 17.8, 4.4, inj, 21.2. With Foster and Charles out for the season, he is performing as a top 5 RB right now.
OK, so how to project forward?
In his past 5 games, he is averaging 6 carries per game at 5 ypc. Is there any reason to think his average carries per game will drop? I don't think so. Is there any reason to think his 5 ypc will drop? I don't think so. His career ypc average before this season was 4.75, and that was playing for teams that did not have the kind of passing game threat that Brady, Gronk, Edelman, et al. pose. The eye test also suggests that his average has as much or more to do with his talent than scheme and supporting cast. So if he plays the remaining 9 games, it seems reasonable to project that he will get another 54/270 rushing, which would bring him to 99/490 rushing for the season.
In his past 5 games, he is averaging 5.6 receptions per game at 10.6 ypr on 8 targets per game. His catch rate of 70% seems sustainable; it is similar to or below other great pass catching RBs like Charles, Bell, Woodhead, and Sproles. Conservatively, suppose his targets drop to 7 per game. With a similar catch rate, that means 5 receptions per game. Conservatively, I could see his ypr dropping a bit to, say, 9.5 ypr. If he plays the remaining 9 games, it seems reasonable to project he will get another 45/427 receiving, which would bring him to 80/776 receiving on the season.
How about TDs? Well, he has 4 TDs in his past 5 games and 4 in his 6 total games. Even though you don't like to do it, I will include the first game with 0 TDs here, since it is the more conservative route. He has averaged 0.67 TDs per game. Projecting forward, this projects to 6 more TDs on the season, meaning he would finish with 10 total.
These projections total to 1266 YFS, 80 receptions, and 10 TDs. This all seems reasonable IMO. You seem to be aligned on catches and rushing attempts, so your only issue appears to be TDs.
As you have pointed out, Lewis has not required goal line touches to get 4 TDs in 6 games. So the fact that he doesn't get them doesn't really seem to be a problem for his TD scoring potential. This is an offense that has scored 28 TDs, and Lewis has scored 4 of them, which is 4th on the team behind Gronk, Edelman, and Blount. Is there any reason to think that will change going forward, i.e., that others on the roster will surpass Lewis as a TD threat the rest of the way? I don't think so. Is there any reason to believe the Pats offense will regress in TD scoring by a significant margin going forward? I don't see why that would happen barring injury to key players.
For you to project Lewis to score potentially as few as 2 more TDs in what you seem to agree will likely be about 100 more touches over 9 more games seems like a reach. You said 6-8 TDs here. If you instead settled on 8, that would seem conservative, but more reasonable.
So where would 1266, 80, 8 rank? In normal PPR, that would be 255 points = 17 ppg. In my PPR league mentioned above, that would have ranked as RB #8 last season, with Charles and Foster ranked higher. With 2 more TDs, it would have ranked as RB #7, with Foster ranked higher.
In standard, 1266/8 = 175 points = 11.7 ppg. Per FBG, last year that would have ranked as RB #13, with Charles and Foster ranked higher. With 2 more TDs, it would have ranked as RB #10, with Charles and Foster ranked higher, and about 1 point behind RB #9.
So it looks like projecting him as a RB1 in PPR, with a chance to sneak into the top 5 but not a lock to do so, is a very reasonable take, but it looks like projecting him as a RB2 in non-PPR is likely understating his expected performance.