Here are some ways to figure out if it was paced right: use your HR meter in tandem with your PM can help determine how well paced the effort was. Watch for big dips in power (even with 10s of smoothing) other than the usual surge right at the start and end. Also watch that your HR comes up to threshold by 3-4 minutes, and holds reasonably steady throughout that 10m. And if you had enough gas in the tank to really go way harder in that last minute, you probably didn't go out hard enough. If your normalized power is way different than your average power, there again you probably went too hard at some point, or eased up too much elsewhere.
As you get better at pacing those efforts you'll find the average power will naturally ramp up as you get better at holding right around threshold.
Also consider whether the 5m 'blowout' interval prior to the 20m interval is leaving you a bit worn out. Depending on your fitness it might make more sense to do a longer and slightly less intense warmup, with maybe a 5m threshold effort in there just to get you ready to suffer a bit.