Time2Num and Timesheets

Hello,
I have the following code for a simple timesheet.  I would like to add in a meal break field that can be taken off a total and am having trouble with the script.

The 4 fields in question are: rosstartmon (this is the shift start time), rosfinmon (this is the shift finish time), rosmealmon (the meal break) and the calculation field which is ResultHours.  All fields are time/date fields other than the ResultHours field which is a text field.

Here is the code I use to work out hours worked

 cqnursetimesheet.timesheet1.ResultHours::calculate - (FormCalc, client)

var TimeCount
var OneMinute = 1/60

if (rosstartmon ne null and rosfinmon ne null) then
	//if start time is lower than end time. 
	if (Time2Num(rosstartmon.formattedValue, "HH:MM") < Time2Num(rosfinmon.formattedValue, "HH:MM")) then
		TimeCount = Abs(Time2Num(rosstartmon.formattedValue, "HH:MM") - Time2Num(rosfinmon.formattedValue, "HH:MM")) /(60 * 60 * 1000)
	//If start time is higher than end time	
	else
 		TimeCount = 24 - Abs(Time2Num(rosfinmon.formattedValue, "HH:MM") - Time2Num(rosstartmon.formattedValue, "HH:MM")) /(60 * 60 * 1000) 
	endif

	var Minutes = Round(TimeCount / OneMinute)
	var Hours = 0
	
	for i = 0 upto 48 step 1 do
		if (Minutes >= 60) then 
			Minutes = Minutes - 60
			Hours = i + 1
		endif
	endfor
	$ = Concat(Hours, ":", Minutes, "")
endif

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How do I add the meal break time into this code so that if the user enters (for example) 30 (for 30 minutes) it will take it off the total hours worked.
Thanks in advance