Getting better at defining north star metrics

I have failed the final round of interviews twice now due to my ability to come up with a good north star metrics.

Feedback from the most recent rejection: 1. Took too long to come up with a north star metric which showed limited experience working with them 2. The north star metric he came up with wasn't very good

It's valid feedback. Having worked as a PM primarily for an enterprise SaaS product, in a sales led organisation, I found it difficult to identify, refine and sustain metrics that helped our team measure impact and success.

My question - does anyone have advice on training courses, reading or practical exercises I could do to get better with metrics/north star so that I don't continue failing this interview step and also so that I'm better at this within my actual work?