Where do I find/set Parameter Name for dynamic population?

I have looked through the documentation and forums but haven't found the solution to my task yet. I have a form that registers new users. I have a second form where registered users will add a list of heirloom seeds they have - so they will be filling out the second form many times and I don't want them to have to re input the same info over and over. For example, I want to pre populate the second form with the user's USDA Growing Zone (and other information such as address, elevation, and phone) which I asked for on the registration form.

I tried this in functions.php:

// populate the field with "usda_growing_zone" as the population parameter with the "usda_growing_zone" of the current user
add_filter('gform_field_value_usda_growing_zone', create_function("", '$value = populate_usermeta(\'usda_growing_zone\'); return $value;' ));

// this function is called by both filters and returns the requested user meta of the current user
function populate_usermeta($meta_key){
    global $current_user;
    return $current_user->__get($meta_key);
}

But usda_growing_zone is simply the field label with underscores, I do not know how/where to find the actual parameter name for that field in the registration form. If you could help me with this I think I could figure out the address, phone and elevation...
Reg. form http://heirloomseeds.alterra-wv.com/register/
Form 2 http://heirloomseeds.alterra-wv.com/add-seeds/