My understanding of the Focus Power Test rules indicates the Librarian gets a bonus to his FP Test equal to 5xPsy Rating to which he's attempting to manifest the Power. So if a Librarian with a Psy Rating of 3 manifests at Fettered his bonus is +10%, and if he Pushes he gets a total bonus to the FP Test of +30%. If this is the case I haven't thought it to be a good mechanic. 2 or 3xPR seems sufficient, 5 just seems excessive. I know multiples of five are intended to make the math "easy", but it's a multiple result that is then added, so the logic fails. Even were the multiplier 2 or 3 it would still generate a number that needs added to the base, and simple math is just that; simple. Psy Rating 3, multiplied by 5, equals 15, add to base WP score of 47 is no harder or easier than Psy Rating 3 times 3 for a total of 9 added to 47. The logic of choosing 5 as a multiplier just fits within the standard 5/10 mechanic presented throughout the game, but otherwise flops. Additionally, Fettering a Power shouldn't make it easier to manifest, in my opinion. Increasing the Target Number increases the chance of successfully manifesting a Power, yes, but also increases the chances for Psychic Phenomena, which penalizes a Librarian just for manifesting Unfettered Powers. Again, an odd consequence, not that PP happen often with Librarians manifesting Powers. I think the rule is meant to act as the FP Test's version of Range/Size/Skill modifier, but it just seems counter-intuitive.
In the end, even though it bothers me, I've come up with no alternative I'm entirely pleased with, so have changed the multiplier to 3xPR rather than 5. So a Librarian with PR 3 that Fetters a manifestation gets a bonus to his FP Test of 6, which is insignificant enough for me as to be nothing, but the Librarian Player still gets something rather than nothing, and Pushing he would get +18 to his FP Test, instead of +30. Four advances in WS only gives you +20, and that's after the expenditure of a significant amount of XP, whereas the +30 from Pushing comes from…uuuhh…making a choice? No. Just no. Sure PP/Perils…but I think all Players, as much as they say they rue PP/Perils, will tell you with all honesty they look forward (with relish) to hearing the results rolled on those tables. They're Space Marines; what's withered plant life to them? Summoned a Daemon? Just another challenge to be faced and bested.
I suggest 3 multiplied by the target Psy Rating as a bonus to the Focus Power Test.