The description for demonic presence says it increases hell portal production by 0.5% for each other building but the percent scales by 0.05%.
Best idle game I've played, by the way (:
There is bug in the current version of the game that makes Hell Portal produce less than intended. We already managed to fix it, will be up in the next update :)
 "Minions"){kind=icon}
 "You Have To Burn The Rope"){kind=icon}
 "Book of Mages: The Dark Times"){kind=icon}
 "Realm Grinder"){kind=icon}
 "Green Soldier"){kind=icon}
 "Reincarnation: All Hallow's Evil"){kind=icon}
 "Birdish Petroleum"){kind=icon}
 "Kids Vs. Ice Cream"){kind=icon}
 "Tower Defence (random!) UPGRADED"){kind=icon}
 "Merlin's Revenge 2"){kind=icon}
 "KOHCTPYKTOP: Engineer of the People"){kind=icon}
 "Take Something Literally"){kind=icon}
 "GeoSketch"){kind=icon}
 "Smokin Barrels"){kind=icon}
 "Take Your Canvas Bags To The Supermarket."){kind=icon}
 "Radioactive Cow"){kind=icon}
There is bug in the current version of the game that makes Hell Portal produce less than intended. We already managed to fix it, will be up in the next update :)