Start with the lens’s angle of view, because that determines the cone/pyramid of light that must pass through the hood without being blocked. A round hood is easiest: the wider the lens’s angle of view, the shorter the hood must be. A petal hood is based on the same idea, but shaped to match the camera’s rectangular frame: imagine a long round hood, then remove the parts that would intrude into the image corners, leaving the “petals.”

In practice, find the X30’s angle of view at the focal length you care about, or compare it with another lens/camera hood made for a similar angle of view. That gives you a starting point for dimensions.

The critical part is avoiding mechanical vignetting, especially at the widest focal length. So design for the widest setting first, make the hood a little shorter than the theoretical maximum, and test it. If you zoom in, a hood sized for the widest end will still work, though it may not be as deep as possible.

If you want a petal hood, derive it from the rectangular image area rather than a circular one.