Automatically set value at registration time?

A requirement for a project I'm developing is that users enter some statistics, like height, weight, age and gender at account creation time.

From this, we want to generate a number that's used in fitness classes.

Basically, we want to have a one-time calculation for the initial value, based on the profile2 fields a user enters, but have it populate another (hidden from the registration form) profile2 field, so that the user can change it later like any other profile2 field in their account settings.

What is the best way to go about this? Can anyone offer a suggestion? I've been beating my head against the wall enough that it's a little mushy now.