How do I Calculate the M in my MVP?

Talk to potential users and customers to get an understanding of where they feel the pain and at what point it will be something really helpful to them.

Talking to 2-3 of your target customers will help you understand how you need to change the idea. And keep doing it until you get consistency in their feedback.

You also get bonus points if they are willing to pay for the privilege of getting access to it first while you work with your first small set of users.

The whole point of M is to design an experiment that answers the very question you just asked. This doesn't necessarily mean you need to do any coding, but might instead be an e-mail sign-up page or even a marketing web site for you product with a purchase page. You need some real indication of whether people might purchase your application ... Not our opinions.

Your links to HN are broken.

Easiest calculation in the world!

M == minimal

1. Do you think anyone would use my MVP or service?

Depends on whether you build it or not. If you don't build it, the probability of its success is zero. If you do build it, the probability of its success is nonzero.

2. Have I missed any other market segments?

Yup. All of them! You miss 100% of the shots you don't try.