Hi, I have a pitcher who is 40/162 Ip, and 8 IP/G stamina.

Game 81, has thrown 20 innings on the season and he has just returned to a 100 rest, game 82 I throw him 8 innings. Game 83 his rest will be in the cellar. However, he'll be effective the entire game 82, because the only relevent factor is his starting rest of 100, and his IP/G. It doesnt matter if by the 6th or 7th inning he is on pace to go way over on season innings, the only factor in his rate of fatigue during the game is his ip/g, correct?

Largely correct. However the sim is based on pitches, not innings.

Elbirdo's post on pitch counts

https://www.whatifsports.com/forums/Posts.aspx?TopicID=154851