Precision feat

[Feithlin]

Where's the 25% fort bypass in your calcs for extra SA and crits? Vs 80% fort foes your 19-20*7 barbarian goes from 31/20 to 21.4/20 and precision helps by putting it back up to 24.4/20.

Similarly, if you have 13 SA damage/hit (which every build should plan to have) then precision adds 3.25 damage/hit vs any foe who isn't sneak immune and with 25% or more fortification (raid bosses, warforged and sneak immune mobs hit by Dark Elusion). Many builds will have additional SA from other sources as well (rogue dilettante/splash, halfling cunning, rogue PLs, ninja-spy, shadowdancer, masked ball or other epic abilities etc).

*Edit: I misread your observation. So here is the answer. I considered it, that's the (1-%F) part. For the benefit of this 25% fort bypass, you need to calculate with your own build. There's no way to evaluate it in a simple way.

I only consider the benefit from PA, so without -25% fort. from precision.
This is to evaluate the benefit of precision for your build, because the value of a hit depends on too many factors, while we only need the weapon profile to evaluate the benefit of PA. If your character has 13 SA damage, it will counted to evaluate the single average hit and check if it's better than what PA is giving to you.

For example, if I want to evaluate a level 20 Kensei III against 80% Fort. The average hit would imply the weapon basis (1.5[1d8] = 6.75), Str bonus (+15 for example), the weapon bonus (let's say +5), the elemental/alignment damage (+7 from holy, +3.5 from lightning), special effects (+9 average from lightning strike), the damage from feats/PrE (+4 feats, +4 enh.), the SA reduced by (%F - 25%) (+8 * 55% = +4.4), sum it all:
6.75 + 15 + 5 + 7 + 3.5 + 9 + 8 + 4.4 = 58.65, apply off hand (same but with +7 from Str and 80% proc) = 40.52, for a total of 99.17.
I would also need to account for the benefit of criticals for the bypassed fort. In this case: 0.25 * ( 17*(6.75+15+8) + 5*2*11 + 14) / 19 = 8.29.
My average damage that doesn't benefits from my damage with PA is 107.46, which isn't enough to bypass the benefit of PA even against 80% Fort.

But it would be very different with a better weapon, ED, etc. All this cannot be appreciated in general. Everyone has to do the calculation for himself. But at least, you get a comparison. A dual wield rogue certainly always benefits more from Precision than from PA, as soon as he doesn't hit on a 2. But it may be different for an acrobat for example, who would benefit more from PA.

[Feithlin]

But does that also count the attack penalty from PA? With the new combat system, it could be more like 2 hits that precision gets more than PA.

No I didn't, you're right. You may then compare the PA benefit to 2 hits instead of one. So in the example I give, Precision would be better if I were hitting on a 4 with PA, and on a 2 with Precision.

[Truga]

In raids I guess it makes sense that if you do 20 or more sneak attack damage/hit then Precision will always be better in raid situations unless fortification is already below 25%.

Basically this.

And the same goes for critical hits. If your weapon gives you an average of +20 crit damage per hit (crit damage, spread across 1-20 instead of x-20), you're better off using precision. The number goes up to +40 when using 2handed weapons, and goes even higher if you roll WF/H-orc (racial PA enhancements) and/or barb (class PA enhancements), but then again, SoS has an insane crit profile:

Max THF PA damage is 22 (horc/WF barb, max PA enhancements) I think. Times 4 that is 88. Add another 15 for sos crit damage from PA (should be 20, but we'll consider the enemy having 25% fort, for no fort you use PA anyway).

103/.3 (sos has a 30% crit range) is roughly 350. So if you're critting for more than 350 with PA off, precision is probably a better choice. For bosses, constructs and undead anyway. And this is without considering any SA.

For a khopesh (20%/x3), it's 8 (assuming not barbarian this time, but still horc or WF), and another 4 from crit damage (assuming some fort still), so 48 is the number. 48/.2 is 240, so if your khopesh crits for more than 240 with PA off, you're better off using precision on fortified enemies.

These are just simple guidelines though, because I CBA calculating right now exactly at which point precision becomes better. And again, these are without counting in any SA damage, which can probably be simply substracted from the modified PA damage "rating" (the x4 one).