The Stryd app does auto-CP, based on your workouts. But you need to make sure that you’re feeding it the right information. Here’s what I would suggest:
1. 3 minute and 12 minute best effort runs. Same day, roughly 30 minutes apart. Just make sure you warm up and alternate easy running and walking in between the test periods.
2. Something longer. 5K to 30 minute range. Also best effort. Not the same day as the 3/12 minute test, but I would try to do it fairly close (within a week or so) in order to make sure that the “snapshot” you’re giving it accurately reflects your current fitness level.
Some people just do do a 3/9 or a 3/10 minute test run instead. But with me, this always results in overestimated CP and has me training too hard. Extending this to 3/12 and adding in something longer seems to dial it in pretty accurately.