When you use a psy power under your base PR, it is said in the rule book :" By minimising the amount of power he draws, a psyker can reduce the chances of horrible side-effects such as Psychic Phenomena or Perils of the Warp...", but i can't find any rule about it which tells if there is a bonus to use a psy power under the base PR.
exemple: I have a PR 4 and WP40 and i use a psy power with an effective PR of 2, so two les than my base PR. i roll a 22 so it's a success but i trigger a psychic phenomena. How the fact that i use an effective PR two points below my base PR is represented? do i have bonus of -20 on the psychic phenomena table ?