If you highlight the dummy and go to properties you'll see the rotation properties (rotx, roty, rotz). Change the one with a number either higher or lower to adjust how far out it opens.
Just as a side note as well, if you want you can use the dummy "Cargodoor" instead of bonnet to make something open whenever the trunk is open and save the bonnet for the actual hood. Just putting it out there in case you didn't know.
Good luck
Thero