Auto extend formula down a column
 

I have data in columnB which is automatically inserted from an input sheet
by a macro running via a button.. In column A I have the following formula
which inserts a unique number to that column should a unique item be found
in column B

"=IF(COUNTIF(B$1:B70,B70)=1,MAX(A$1:A69)+1,"") ".

How can I have the formula automatically inserted into row 71 with the
formula updated for that row, ie

"=IF(COUNTIF(B$1:B71,B71)=1,MAX(A$1:A70)+1,"") ".

Sandy