Lake Girl
Moderator
Many go with 4" in a basement to accommodate the additional elbows used to get venting above grade but it sounds like you have at least one wall that has a better portion open. EVL is calculated as horizontal pipe 1'=1, vertical pipe 1' = .5 (1/2), 45 elbow = 3, 90 = 5. If EVL is more than 15, 4" used. If elevation is above 3000, use 4" (realize it doesn't apply to Michigan but added for clarity).