Not really.
The adjust palette has a straighten tool that allows you to rotate the photo. The tool is controlled only by a slider. While rotating a grid is overlaid on the photo, so you can compare your photo to the grid as you move the slider. There is a number readout beside the slider that ranges from -10 to 10 as you go from left to right on the slider (counterclockwise to clockwise). I don't know if the number relates to degree of rotation, or percentage, or any real measurement at all. But you can't go farther than 10 units at once. You can save the rotated photo and rotate it again, but since iPhoto automatically crops the straightened image you'll find that it gets progressively smaller and smaller. After doing one photo 3 times, I lost most of the vertical dimension of the photo. Very unsatisfactory.
Photoshop Elements 4 has a straighten tool where you draw a line across the photo, and the photo is rotated to place that line in the horizontal. There's no control by degree there, either. But it does give you the choice of cropping the photo or enlarging the canvas, so you don't have to lose a chunk of the image.
Also in Photoshop Elements 4, under Image > Rotate > Custom, you
can type the degree of rotation. It sounds like that is what you want.
Regards.