Time fields are number fields that record time since midnight in seconds.
Make Work Hours Lunch a number field.
Then the following two calculation options will compute your total time worked:
WorkHours End - WorkHours Start - WorkHours Lunch * 60 //if Lunch time is entered in minutes (such as 30)
WorkHours End - WorkHours Start - WorkHours Lunch * 3600 //if Lunch time is entered in hours (Such as 0.5)
You can select time as the return type to show time worked in hours and minutes or you can divide the result by 3600 and select number as the return type to compute the time worked in Hours.
Note that if some start times come before midnight and some end times come after midnight, you should make your start and end fields, fields of type TimeStamp instead of time.
Works great. Thank you.