xenon total dmg vs attack

To figure out how much %boss is equal to 1% ATT, you put your stats in the formula. You said you have 280% boss damage, but you didn't mention how much total damage or %ATT you have, so you'd do something like this (I'm going to assume you have 60% total damage (you probably have more, Xenon has a lot of innate total damage iirc. Demon Avenger/Kanna link skills, character cards, hyper skills that add %damage, stuff like that) and 0% ATT).

Formula:
1 %ATT = (100% + %Boss + %Total damage) / (100% + %ATT) %Boss

Your stats (roughly):
1 %ATT = (100 + 280 + 60) / (100 + 0) 4.4% Boss

So for you, 1 line of 12% ATT is equal to 52.8% boss damage. I improvised with the stats because you didn't include everything though so you'll need to find those for yourself if you want it to be accurate.

does this apply too for classes with insane weapon multipliers? (bucc/NL) I mean, a NL with 1000 attack gets more range bost than a shad with 1000 attack O_o

I always sux at this parts of the game (math sux )