cocoa touch - Where is the best pIace to handle repeated casting of variables? (UITextField -> math calculation -> UILabel) -

i creating app takes user's body measurements, , calculates body metrics user inputted measurements.

i have view controllers classes collect user data via uitextfields (which stored nsstrings), passed class calculates data (needs nsnumber). model class passes result view controller displayed in uilabels (which needs nsstrings). no issues, no problems code.

my question is: there common design pattern where handle casting? data collected, inside model class calculation, create class handle casting?

i know of these options available me, makes more readable , grokked code?