Lunch Break Timings Calculations

I need to be able to sort out wether my employees have taken too long on their lunch break

what I need is a formulae that works out the following

No lunch would show a 0
lunch between 0 and 30 mins would show a 30 mins deduction
lunch over 31 mins would show a 45 mins deduction

I am struggling to get a formulae to do all three conditions

i would if anybody could help

many thanks if you could

Paul (Dewsbury of England)