You'll need some text mode. Try something along this line to get started:
displayname=Column Name Here
textmode=true
valueexpression=SUB(56,{actualWork})
valueformat=HTML
actualWork = Actual Hours
I can't remember if actualWork comes through as hours or minutes, if it's minutes you may need some extra math:
displayname=Column Name Here
textmode=true
valueexpression=SUB(56,({actualWork}/60))
valueformat=HTML
workRequired = Planned Hours