[NoOne]

look at errors by position at the end of the year. if you want 10% more, add 10% to the existing modifier. 1.200 would go to 1.320. i say by position, because you might not want to apply a blanket 10-20% increase in errors equally amongst positions.

if you're not too picky, you could get this pretty well tailored in a couple seasons. you'll have to keep an eye on it if you don't use a larger sample size. if it swings more than .0020 you may need some slight adjustments (being a bit more liberal than above due to increased errors relative to my testing). use a restored backup of hte league you intend to use.. so you know it will translate and work.

increasing errors will definitely accomplish what you want. don't be afraid to do +50% / 1.500 LTM and such for some positions if the results warrant it. at the fielding rate you gave, you will have alot more errors.... spread it out how you want it.

an idea that may not work:

will auto-calc, after importing a historical year that should match your target Fielding average, set the errors appropriately? if so, take a screen shot... then another screenshot once you re-type it all in with new errors LTM from auto-calc remaining. you'd probably want to re-import the year you *actually want things to mimic to be safe, then re-type whatever deviates from screenshot.


EDIT:

if you are pre-2016, i think no matter what, things slowly change a bit. even if you import from "2050." expect it to contine to rise into the 90's to 00's before leveling off. you may have to just slowling increase errors a bit each year or a chunk every 3-5 years etc.