IF parameter statement merged with time format

I am trying to do an IF statement based on the parameter (Average or Total). However I am struggling with how to put the two together.

=Floor(Sum(Fields!Avg_ACD_Time.Value)/ 3600) &":"& Format(DateAdd("s",Sum(Fields!Avg_ACD_Time.Value), "00:00"), "mm:ss")

=IIF(Parameters!ReportType.Value="Average", Sum(Fields!Avg_ACD_Time.Value)/CountRows(), Sum(Fields!Avg_ACD_Time.Value))

Many thanks