There are several macros posted on the first page so you'd have to be more clear.
Originally Posted by Brage
If there is any doubt in your mind, use my modified macro which automatically shifts based on target level posted at the end of page one, reposted here. This macro doesn't automatically shift for if you are a Night Elf so you'd have to manually alter it by changing the 5 assigned to "m"iss to a 7 (simply not enough character space).
Target something and it will tell you your avoidance and unhittable percentages. So if you want to measure it against a boss, target a boss and run the macro. If you want to test it against an 85, target an 85 and run it. Bare in mind buffs and debuffs may actually alter the mob for the specific encounter, such as Nether Scion's debuff on Halfus Wyrmbreaker increases his chance to miss by 25% (upping it from 4.4% to 29.4% miss rate against a max level player). Other mechanics may also alter this value, for example a knockdown, or if the target was behind you would adversely affect your values.
/run u,t=UnitLevel("player"),UnitLevel("target") if t==-1 then t=u+3 end ld,d,p,b,m=0.2*(u-t),GetDodgeChance(),GetParryChance(),GetBlockChance(),5 DEFAULT_CHAT_FRAME:AddMessage(format("Lvl: "..t.." Unhit: %.2f%% Avoid: %.2f%%",d+p+b+m+ld*4,d+p+m+ld*3))
Using this macro you are also not on a 102.4% scale when comparing yourself to a boss. It shifts the scale so that you are always on a 100% scale so it is easier to understand exactly how many attacks will get through.