1. Near your curved surface (and in an appropriate plane), draw a shape that equals your desired cutout.
2. Extrude (or push/pull) the shape through the curved surface.
3. Select the curved surface and the newly extruded shape, then do an edit | intersect | intersect with model.
4. Erase the unwanted surfaces to finalize your cutout.

[ c ]

Thats actually what i've been doing so far. I was hoping there was an easier way, or at least a way to calculate the cutout so it doesnt look distorted.

Thanks!! 

Could you expand on your thought, "a way to calculate the cutout so it doesn't look distorted"? No doubt an example would help clarify.

[ c ]

Sure, not a problem.

Easiest way to describe it would be this- take a piece of paper and draw a circle on it. Now take that piece of paper and wrap it around a soup can. While flat, your circle looks like a circle. But once you add the curve to the paper, it ends up looking more like an ellipse.

does that make sense? 

If you were to follow the steps above, starting with 1) a soup can, and 2) a circle on a vertical surface, don't you get what you need by projecting the circle through the can?