My spec, beast mastery, typically uses slightly different builds for a true single target raid encounter (i.e, Lockenstock) vs. one with intermittent adds (i…e, Vexie). Clearly the “Raid” gearing strategy is appropriate for the former. But what are people using for the latter? Looking under the Custom hood, I get the impression that the Mythic/AoE choice boils down to constant 1 target vs constant 5 targets, or customize lets me pick a different fixed number than 5, but what I really need is a ratio of say 2 minutes of 1 target and 15 seconds of 5 targets. Is there a way to do that?
At the moment we don’t “blend” different fight types together. We recommend that you optimize around the most important part of fights – usually the result between single-target and AoE optimizations isn’t so different that you become ineffective on the other.
So if the AoE part of a fight is the most important (killing those adds fast is the most consistent way to win), we usually recommend optimizing around a higher target count. (Or if there’s a lot of AoE and you want to get high-ranking parses 
) But if the AoE is short and infrequent, using a single-target optimization might work out better – you’ll still do decent AoE damage, but you’ll pump out more damage on the boss.