Total Mnutes Worked Showing Negative or Incorrect Value

I have 5 columns respectively; SalesPerson, Minutes Worked, Call Gaps, Minutes Deducted and Total Minutes Worked. Call Gaps column is subtracted from a salesperson's minutes worked. This should show in the Minutes Deducted column. The Total Minutes Worked should show either a 0 (ZERO) if the salesperson didn't work that day. If a salesperson reported only for 1 hour = 60 minutes then had a Call Gap of 10 minutes then his Total Minutes Worked should show as 50 minutes. The Minutes Deducted column should show as 430 (480 (complete shift) less 10 minutes (Call Gap)). Please help. Thank you in advance.

Thank you Kaper