Macro / VBA To Use 2 Columns In 1 Sort

I am trying to write a Macro to basically do a lot of redundant formatting for me and I am stuck on one part of this code. Everything else works just dandy, but I am having the darndest time getting this Macro to sort by 2 columns instead of just 1.

I did the basic first step and recorded myself sorting the datasheet by both columns in 1 sort and got the code, but the problem is that the number of rows will change day-to-day, week-to-week, and so on, so I need it to not specify a cell number as the end of the range.

While searching around, I saw a very handy little piece of VBA code that will search for a header name, which is really nice because that would make it much easier to share this Macro with my coworkers who have different reports that all have columns in different locations.

So, I have a multi-part question. I will start with the most important one:

How do I alter this to not specify A9590 and D9590 as the last cells in those ranges?

ActiveWorkbook.Worksheets("Date XREF").Sort.SortFields.Clear
ActiveWorkbook.Worksheets("Date XREF").Sort.SortFields.Add Key:=Range( _
"A6:A9590"), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortNormal
ActiveWorkbook.Worksheets("Date XREF").Sort.SortFields.Add Key:=Range( _
"D6:D9590"), SortOn:=xlSortOnValues, Order:=xlAscending, DataOption:= _
xlSortNormal

[code]....

Is there a way to have the Macro find where the header row is? The header rows in our reports can appear anywhere from Row 1 to Row 8 depending on how the report is set up (usually we have some information about the data, date range, etc at the top).

Dim c As Integer
With ActiveWorkbook.Worksheets("Date XREF").Range("A5").CurrentRegion
c = .Find(What:="Assignment ID", After:=.Cells(1, 1), LookIn:=xlFormulas, _
LookAt:=xlPart, SearchOrder:=xlByRows, SearchDirection:=xlNext, _
MatchCase:=False).Column
.Sort Key1:=.Cells(1, c), Order1:=xlDescending, Header:=xlYes, _
OrderCustom:=1, MatchCase:=False, Orientation:=xlTopToBottom
End With