I require help with a formula please.

I have the following:

=IF(ISERROR(MATCH(old!$C$1:$C$" & overallLastRow & ",new!$C$1:$C$" &
overallLastRow & " ,0)),old!$C$1:$C$" & overallLastRow & " ,"""")

This formula uses 2 lists:

old!$C$1:$C$" & overallLastRow & "
AND
new!$C$1:$C$" & overallLastRow & "

Obviously old and new are the names of the Worksheets on which these lists
are found. These are direct references to these sheets.

What I want to do is give a sheet a name and link to this in a similar way
that I have given a cell number a name of "overallLastRow".

Can anyone tell me how to do this?

Thanks
Dave