How to position for your first image
When approaching an extremity like a hand or wrist, sight the intensifier in line with the area of interest as it goes under the table.
Adjust cross arm
Next walk around to the side of the intensifier and drop an imaginary line down from the tube aperture. If this is not centered go back to the C-arm and use the forward-back cross arm lock to move the appropriate amount in the right direction. If the centering looks to be more than five centimeters off either way move the whole machine in the desired direction and check again.