Frame adapters for Thule 968 Freeway rack?

[Fetchez la vache]

I'm going to have to get a couple of frame adapters for the Thule Freeway 968..

Am I right in thinking that so long as the distance between the seatpost and stem is between 570 and 670 mm then I can just go for the cheaper 981 version, or am I missing something?

981 Adapter
982 Adapter

cheers