Your formula doesn't work because
[Rule operator] is a range itself, so if your formula is entered in a cell on row 5, Excel will evaluate your formula like this:
=MATCH("THEN",[@[Rule operator]]:[Rule operator],0)
Because A5 is in the middle of the range A2:A12, Excel ignores it and just uses A2:A12.
To address this you can keep the
[@[Rule operator]] bit but you need something to work out where the last cell in your column is.
You can do this by counting the number of rows in your table using
ROWS([Rule operator]), adding 1 for your header row (add more if you have other rows above your table), then using the
INDIRECT function to assemble that into a cell reference. This gives you:
=MATCH("THEN",[@[Rule operator]]:INDIRECT("A"&1+ROWS([Rule operator])),0)
Because this will output a
MATCH row position including the row you are running the formula from, you'll need to subtract 1 to calculate the Offset of THEN value you need for each row:
=MATCH("THEN",[@[Rule operator]]:INDIRECT("A"&1+ROWS([Rule operator])),0)-1
That's it :)