Any idea of the general impact of an extra half deck of pen on an 8 deck game?

Playing an 8-decker that, besides the 8 decks, is a great game. Currently trying to calculate risk on CVCX but penetration only allows 7.0/8 decks. This game has 7.5 deck pen. I noticed that, when starting at 5/8 decks pen, each half deck of additional pen takes off 3-5% of risk? Would this generally apply going from 7 to 7.5 decks as well?