According to a potential list site, at legendary, you can get 40% boss damage and +12% atk.

So here is your balance of stats between att% and boss damage% at its simplest:

Boss Damage%               Atk%
.4                                  .12
.8                                  .24
1.2                                .36
1.6                                 .48
2                                   .6
2.4                                 .72
2.8                                 .84
3.2                                 .96
3.6                                 1.08
etc...

Obviously, you cannot have less than 0% att (which is at 1+0=1.00). So let's look at the ratio after atk% rises to/above 1.  Ideally, according to the chart, when you reach 3.6 or 360% boss damage, you should have 1.08 or 8% atk. But of course, you can get 12% atk so you just try to match it up as close to this as possible.

Basically, it's saying you do not get att% until you have 360% boss damage. So I'll prove the chart is correct and that you should only get atk% after getting 360% boss damage.  Simply put, if you were at 360% boss damage, and you had the choice of either getting 12% atk or 40% boss damage, what would be the better choice?

With +40% boss damage, you go from 360%-> 400% boss damage and that is 4.0/3.6=1.11 or about +11% damage increase. However, since we so far have 0% atk, and if we choose to add +12% atk instead of +40% boss damage, we get a flat +12% dmg which is > than the 11% damage increase.

But it's a little bit trickier than that. Since you basically the formula is 1+(boss dmg%), it's kind of like saying you get a base of 100% boss damage (becuase if you had 0, you do 0 damage [0xanything=0]) . And same for atk% (1+atk%), we start with a base of 100% atk. So basically, from the chart, you need to minus 1 from your boss damage% and from atk%. 360% or 3.6-1=2.6.

So really, at 260% boss damage, you should get 8% atk. At 300% boss damage, you need 20% atk. At 340% boss damage, you need 32% atk, etc...



Here's the new chart that shows those ratios:


Boss Damage%               Atk%
2.6                                 .08    (at +260% boss damage, you should have 8 atk%)
3                                    .2     (at 300% boss damage, you should have 20 atk%)
3.4                                 .32    (at 340% boss damage, you should have 32 atk%)
etc...

So I'll prove this chart is correct.

We have 340% boss damage. The chart says I need 32% atk. But let's say you think I'm wrong and think that 40% boss damage is better and that we should get +40% boss damage instead of another +12% dmg. So in that scenario, we would get 380% boss damage and 24% atk.

Basically, 340% boss damage and 32% atk vs. 380% boss dmg and 24% atk.

damage = (STUFF) *  (1.0 + %att)) * (1.0 + [total boss dmg%+total damage%])

340% boss dmg and 32% atk =  (stuff)*(1+0.32)*(1+(3.4)=stuff*5.808

vs.

380% boss damage and 24% atk = stuff*(1+0.2)*(1+(3.8)=stuff*5.76

So obviously stuff*5.808 > stuff*5.76

So you just follow the chart, and your damage will be optimized.

Note that total damage adds to boss damage, and so do dmg% from skills. You should just try to match your stats as closely as possible to the chart.