The "Up" tilt conflicts with the prism housing on the A900 and limits tilt to about 5 degrees.
Removal of a small amount of metal at the edge would increase tilt to about 10 degrees (maximum of the adapter).

Up tilt would be used only in the case of near/far at the top of the frame, say a ceiling (not often used and 5 degrees is enough for this).

Rotated 180 degrees, "down" tilt is unlimited to the max of the adapter, 10 degrees.
Down tilt is used for near/far at the bottom of the frame such as scenic s.
10 degrees is only used for the most extreme low angle shots.