I bought these for mounting the 95mm rings for my FSQ-85 to a 350mm Losmandy type dovetail. These spacers are needed as the dew-shield and the focuser would be in the way for longer dovetails. As the FSQ with imaging equipment is very back-heavy longer dovetails are more convenient for balancing.
These spacers come with longer screws, so mounting the tube rings to the dovetail was easy and the PLUS system makes it very solid.
If you want to mount a longer dovetail on top of the FSQ then higher spacers would be needed, so it can pass over the focuser-lock.
I think the 30mm would be best to still be able to use the focuser-lock mechanism.