range problems

I am having problems with getting a range command to work. I have a worksheet with 6 tabs labelled SUM, 6401,6402,6403,6404,and 6405. I have a macro that loops through each tab in arriving at a total I put into SUM.

To add the right columns in each tab, I use this command to set the range "startRange" several times in each tab:

ActiveWorkbook.Names.Add Name:="startRange", RefersToR1C1:= _
"='6405'!R58C3"

The command above creates a unique range in the worksheet '6405'.

Instead of using the name '6405' I want to use a variable name to increase the efficiency of the subroutine like this:

MasterFocusSheet = "6405"
or
MasterFocusSheet = ActiveSheet.Name

Then use the command:

ActiveWorkbook.Names.Add Name:="startRange", RefersToR1C1:= _
"='MasterFocusSheet'!R58C3"

When I do this, I get the error message:

"Method 'Range'of object '_Global' failed"

Can anyone help me get around this?

Thanks