It should be noted, that the AoE Cap was recently increased to 20 targets if i recall correctly. Apart from that, very useful calculations, thanks!Oh no you don't, you asked me a maths question and that means it's time for theorycraft.
According to my current sims my average howling blast hits for 247400 on the primary target, so 123700 on every other target.
So on n mobs where n is not greater than 10 my howling blast does 247400 + 123700 * ( n - 1 ) damage, which can be simplified to 123700 * ( n + 1 ), and the maximum damage is 1360700.
Also according to my sims, my average blood plague tick is 23413.
So pestilencing blood plague to n extra mobs is worth 23413 * ( n - 1) * t, where t is the number of ticks they stay alive for.
So what we are looking for are the cases where 123700 * ( n + 1 ) is less than 23413 * ( n - 1) * t.
We can work that out as follows for values of n that are not greater than 9.
123700 * ( n + 1 ) < 23413 * ( n - 1) * t
( 123700 / 23413 ) * ( n + 1 ) < ( n - 1) * t
( ( 123700 / 23413 ) * (n + 1) ) / ( n - 1 ) < t
It's a lot simpler when looking at values of n greater than 9. Due to the aoe cap, the maximum damage of a howling blast is roughly equivalent to 59 blood plague ticks. So you need 1 tick on 59 or more targets, 2 on 30 or more, 3 on 20 or more, 4 on 15 or more, 5 on 12 or more and 6 on 10 or more.
So let's look at the cases for smaller numbers of targets. As I said you're looking for when ( ( 123700 / 23413 ) * (n + 1) ) / ( n - 1 ) < t.
I'm not going to write the whole table out for every number of targets, I'll just give you my results.
You want 6 ticks on 9 or 8 targets, 7 on 7, 6, 5 or 4 targets, and 8 on 3 or 2 targets.
Realistically speaking those numbers are irrelevant for small numbers of targets as you could just tab your plague strikes onto the extra targets anyway, but the larger numbers should be reliable enough.
Of course there are very few situations with enough targets or adds living long enough for any of this post to be relevant anyway. So if you need to ask just don't pestilence. Apart from during the terrace intermission at garrosh, maybe. But probably not.