30/30s are about repeatability, and they drain your anaerobic capacity and turn the efforts into more of max aerobic power workout. Going to something like 13x30/15 push even harder on your aerobic system.
Anaerobic capacity (and max aerobic capacity) is different for everyone, and therefore as you mention the power target and the number of intervals will vary. My personal belief is that a workout like this requires finding your max repeatable power/effort (its not all out like the workout below).
One coaching consideration - increasing anaerobic capacity before working on repeatability. A capacity workout would look like a (near) max effort 30-60 second effort, followed by a full recovery which might be something like 6-8 minutes, and then repeating for a total of 6-8 efforts.