(I'm not sure how useful this is, but it gets my mind off the pain during hill repeats...)
When I'm doing ZCI™s, especially when I'm just getting started in a threshold phase, I will do the first 5' interval without power (set on max), and just pace it by feel. Then I use the resulting average from that effort as a target for the rest of the workout.
Before I had a power meter, I did a lot of work on my RPE interpretation. I would do hill repeats with negative time splits. I got to the point that I could do six ~5' repeats consistently dropping time, but keeping all the times w/in 20s of each other, and that included an absolute all-out effort on the last one.
I did a bit of coaching, and would take the athletes on hill repeat rides. For the inexperienced riders, I wanted to help them learn to pace, so I would ride next to them and try to get them to go a tiny bit easy on the first one. During this first climb, I dialed in my leg sensations so I could "play back" the effort for the remaining climbs. I was consistently able to pace them to the same kinds of times for the workout, ending with a really hard effort on their last one.
I continued this pacing when I got a power meter, and found that my power was doing the same thing, very close, but always improving. Then I was able to dial it in even tighter using power. Once I'm in the hill repeat groove, I can shoot a tight group of repeats all w/in 10W of each other, each one 1-2W higher than the previous, ending with an all-out effort that slots right in at the top of the 10W range.