In '72, the closest NL team to .500 was the St. Louis Cardinals. They went 75-81 (everybody played only 155 or 156 games that season, but I can't remember why). At 6 games under, they were the closest in the league. 6 teams that season were well over .500 and 6 were well under.
If you substitute Lefty's 41 starts for for the 33 that Reggie Cleveland threw, plus the 8 from Don Durham, maybe you get close to your question. St. Louis was 2-6 in Durhams 8 starts, 45 innings worth. They were 16-17 in Cleveland's 33 outings, 231 innings. They were clearly better than replacement throwers as their FIPs were better than the league average, although B-R gives them a combined WAR of fractionally below 0.
So St. Louis was 18-23 in their 41 starts; they combined for 276 innings.
For Lefty to have been worth just 12 games to St. Louis that year by replacing the 41 Cleveland/Durham starts, the Cards would have had to go 30-11 in his hypothetical replacement outings. .Remember that Philly was 29-12 in his 41 starts there. Philly was 11th in the league in runs scored in '72, St. Louis 8th. St. Louis scored 67 more runs that year than Philadelphia. Philadelphia score 0 or 1 runs in 8 of Carlton's 41 starts and just 2 runs in 9 more. In 40% of his starts he had support of 2 runs or less. His average support was 3.83. Throwing for the Cardinals, Bob Gibson got 3.93 runs of support per game, with 2 runs or less in 12 of his 34 starts, about 36% of the time.
Looking at all that, and considering the extra quality innings that Carlton threw, I would say it is a safe bet that he would lift St. Louis to 87 wins, a 12 game improvement. It is likely a bit more. If you figure the extra 70 innings, then you probably get even more than that.
Interestingly, Carlton had, of course, played for the Cards prior to '72. He was shipped to Philadelphia becasue of a contract dispute, the Cards getting Rick Wise in return. Wise was a 110 OPS+, 2.92 FIP, 16-16 pitcher for St. Louis in '72, throwing 269 innings in 35 starts. The Cards were 18-17 in his outings, he averaged 3.71 runs of support. B-R says he was a 5 WAR player. This suggests that the Cards would have won 7 more games with Carlton than they did with Wise. Considering the extra 80 innings and the 2.01 FIP that Carlton had, I suggest it would have been one or two more than that. The Cards would have had to go 25-10 in those 35 starts to pick up 7 games. That's a 71% rate, the Philles were almost exctly that in Carlton's 41 starts. Considering the Cardinals were less than a .500 team, it is "unlikely" that whey would have won 4 of the 6 starts that Carlton would have got, above and beyond those of Wise. So I think it is fair to say that Carlton was likely 9 games better than Wise that year. Carlton doesn't get St. Louis to the playoffs that season (Pittsburgh's 96 wins takes the division) but it does get them to the 3rd best record in the league, up from 7th.
Weirdly, all that St. Louis/Philly comparison works pretty well.
And I will admit, they is some conjecture, wool of bat and tongue of dog in my analysis, too.