How to use time in If statement

B C D E F G H
Monday
Start Finish hrs Start Finish hrs
1 Head Cehf Mirko Gill F/T 18:00 {0}:30 6:30 15:00 {23}:00 8:00
2 18:00 {1}:00 7:00
3
4
5
Excel 2013
As a teacher I am writing a rostering spreadsheet for my students to use in assessments.
I am trying to write a formula for colom 6 "Hours" (H) which will automatically deduct 30 minuits from the total if the staff member has worked over 6 hours (meal break)
I have tried all possuble formulas that I can eg. If(G3-f3>=6:00,G3-f3-:30,G3-F3) I vave tried most other combinations . can anyone assist me please

Peter Braun