Sorry if I wasn’t more clear sooner on this topic. I’ll elaborate a little more now.
Generally the 1/2 of focus duration is a general rule of thumb that works for many people. As you spend more time at the recommended power, MPA will decrease more (XSS/hr will increase, as will difficulty), so you may need to spend overall less time at the recommended intensity. In practice, this would mean less repetitions of longer intervals. This might be good if you’re able to handle the difficulty & strain, perhaps late into the built phase. However, athletes with a lower training load or athletes that are earlier into a build may not be able to handle that harder effort, so it may be easier to break it into more repetitions of shorter intervals, if that makes sense.
I also briefly touched on this topic on the Facebook group, where I stated that the other key to the equation is specificity. Focus looks at the accumulation of low, high, and peak strain and provides the resulting focus duration. Specificity looks at how that strain accumulated matches the work allocation ratio. Think of it this way, if you noodle along for 1 hour and then sprint all-out for 15s, you might end up with a focus duration of 1:00. But the ride didn’t really train your 1:00 power, because none of your efforts were performed at 1:00 power. This would have a very low (polar) specificity rating, compared to a ride that had a pure 1:00 focus, which would require you to spend a significant portion of your ride at/near 1:00 power.
Hope that makes a little sense!