Creating calculations with user data

Hello,

So i'm trying to make this app for my fitness company, where people can sight in, add their personal data and the app will calculate, for example, your BMI.

The idea is that after you sign up, you get to fill in a couple questions:

After you fill in your personal data, you get to the 'home screen' where it has calculated your BMI. You can change your data (using the pensil), and it shows a graph that tracks your weight after you change it:

I figured out how to pass data from the first to the second page, but im stuck on using the data to calculate, for exmaple, someone's bmi.
I can't find a proper tutorial for this. Is there anyone that can help me with this?

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