You can use any club to determine "2 club lengths"
For free relief from an obstruction you should use the club you intend to hit.
However if you then decide to use a new club, perhaps a better lie, if you are not clear with that new club you can use it to gain relief (and so on and so on, until you are hitting your driver off the deck)
You then do not have to continue to use that club.
so you can go
8I -> 5I -> Driver
and then hit an 8Iron from the Relief point with a driver.