It's not THAT complicated. I have a mental process for it but I'm sure Cyborg or North know how to do it with real math.
Analogy from school - your grade in a class changes by the percentage the new mark is worth * the difference between the new mark and current mark.
If you have a 70 and you get 100% on something worth 10%, your grade goes up 3%.
If you have a 70 and you get 79% on something worth 1/3 of your final grade, your grade goes up by.... 3%.
Now consider your incumbent mark to be the league average pitching ratio.
Consider the new mark to be the specific player's ratio.
Consider the percentage the mark is worth to be the innings / week that they give your team.
It's difference from league average * proportional contribution to your weekly innings.
So the calculus changes based on roster construction. If you own more or less SP or RP it changes the thought process. One elite RP doesn't do as much for a team with 10 starters chugging innings. One league average-ish SP heavily sandbags a team that relies on strong relief ratios.