Two things to consider: everyone is different, and you may lack anaerobic training time.

Different interval lengths have different effects. The more usual thing is to find FTP then do intervals of a specific length at a specific percentage of FTP. So the first thing you want is to decide what you're trying to improve or optimize and then decide what interval protocol is most likely to accomplish that. It seems to me that you're going about it bassackwards.

Rather than start with a number and then use it for intervals, my usual method is to figure what interval length I want to do and how many reps, then find by experience what my output should be to be able to do the reps and then fail to achieve that output on the next attempted rep. Hopefully that output goes up after a few sessions of work and a recovery period.

I realize that many people put a lot of stock in their numbers. I'm more inclined to do the work and have less formal testing. I can tell pretty easily if I'm getting faster. But that's just me.