How would i go about combining this string?

=IF(B43=Medicaid_Medical!$A$2,COUNTA(Medicaid_Medi cal!$A$4:$A$50)/COUNTA(Medicaid_Medical!$A$4:$A$50),0)

I need all of the A's to change according to the column they are
representing. I have A - Z in a column that I would like to reference so
that when I drag down the letters change accordingly. I know the row #'s will
change without the $ signs when I drag down, but I need the column letters to
change as I go down...

When I try:

=IF(B43=Medicaid_Medical!"&I43&"$2,COUNTA(Medicaid _Medical!"&I43&"$4:"&I43&"$50)/COUNTA(Medicaid_Medical!"&I43&"$4:"&I43&"$50),0)

Replacing the A with the cell that has A in it, it does not work... Is there
another way to do this? Transpose, somehow?