I am not sure if this information is avaible anywhere already. I couldnt find it so I decided to do some testing. I learned a few things and thought I would chare them with the community.
Few things are shown on this chart. The x-axis is Finesse / -Resist Rating, the Y-axis is % Reduction in Resist Chance.
Obtaining -Ressit Chance % To get the data points for the LM -resist rating (debuff) legacy set I unequiped all gear with finesse then made sure i had no traits equiped that improved any debuff skills (eg fast loader) and then went to the wailing hills and mezzed a troll and continued to spam debuff skills on a perma mezzed troll forever (mostly SoP:Command/See All Ends, Fire-Lore, Frost-Lore, & Dispel Corruption). Each data point represents over 1000 skill uses. I used multiple level books (from 53-75) 3rd Age and 2nd Age to cover the wide range of - resist rating.
Finesse Then (with some help) I collected sets of data for the finesse rating and its associated -resist chance. As you can see this fits well with the collected data.
Diminshing Returns Curve
From other areas on the forums i know that the lotro systems guys like to use a form of
for a lot of rating conversions. This same type of equation is used for outgoing healing (also with k=400). Phys & tact damage are a bunch of these types of curves with different K values over differnt ranges of rating. So I assumed this form of equation and played with K until it fit the data and the finesse curve. The blue curve seems to be a good fit with K=400.
What does the corelation mean?
Well it means that -resist rating legacies & traits follow the finesse curve. This is probably true for all classes (-resist riddle trait for burgs, -cry resist for guards, etc). Also, you notice there is not a whole lot of diminsihing returns (DR) applied to finesse at typical levels we can obtain in game.
How often do mobs normally resit my attacks? Using Knowledge of the LoreMaster (KotLM) I inspected seveal mobs (without any finesse or -resist ratings equipped) and I performed the same testing to get the baseline resist chance. The trolls in LLG have 'fair' resitance for tactical and physical attacks. The ressit chance (after over 1000 skill uses) was 9.10%. I then went into OT t2 and inspected the wargs which are lvl 77 and have 'Good' ressitance to tactical skills. Luitenants in solo lvl 75 tier 2 skirms also have 'Good' resitances to tactical/physical attacks. In tier 3 the luitenants have 'Superior' resistances. I then repeated the baseline resit testing (no finesse or -ressit gear/tratis equiped) on mobs with differnt levels of resistance. The resulats are summarized below. The 'N' value above the bar is the number of skill uses.
I believe that 'fair' is probably closer to 8% and 'Good' closer to 10% as the gap between the 2 really isn't that much from my testing. Superior is probably closer to 12%. I am not too concerned with ratings less than 'Fair' as they probably won't matter that much. I also assume that 'Remarkable' ressitance to tactical/physical attacks is possible but only on bosses where cc will not matter (but debuffs will).
How Much Combined Finesse / -Resitance Should I Have? Here is a table of the type of mob resitance (obtiaed from KotLM scans) and the associated combined finesse/-ressit needed to get close to zero. Im not sure how overloading finesse/-resist works. More is always better but I consider these as minimums, at least for trash mobs ('Good', & 'Superior').
Final Thoughts The trash mob level in OT t2 is 77. This can be adjusted for in the equation (i believe) by setting lvl to 77. This drops the curve but not by very much (eg from 9.1% to 8.9% reduction at 3k Finesse).
I am not sure if the take-aways from this applies to B/P/E. Unfortunately, KotLM does not give us a B/P/E 'grade' of the mob. We may be able to infer from physical resistance but that might be a stretch. At least for me, I will use these numbers for my melee/ranged classes as a goal.
So to summarise, having 5K finesse would mean very little benefit from the book legacies.
Or alternatively 3 to 4K finesse with a book with all 3 legacies would allow you to be very efficient with the legacy points spent I.e. to achieve a total of about 5K.
Fascinating, might make me tweak my LI points around on my 3rd age Debuff book where points are very tight.
Oh yeah, I'd assume stacking, but I was curious if they stacked by adding the ratings and then convering to a percentage decrease, or if each was converted to a percentage decrease independently and then added together.
Also, there's only been six values for k so far. 1190 / 3, 2380 /3, 1330, 150, 350, and 300. Given that that covers mitigations, damage bonuses, criticals, devastating criticals, finesse, resistances, avoidances, partial avoidances, outgoing healing, and incoming healing, they get reused a lot.
Oh, you might want to check your error bounds - I'd use margin of error. (Well, okay technically I'd use an inverse Beta function, but they work out to be pretty similar.)