1. A method for adjusting a milling rate of a focused ion beam in a cross-sectioning operation, comprising: providing a sample having x, y and z dimensions with a chevron on a surface of the sample defined by the z-x plane, the chevron having first and second branches each extending at a same predetermined angle from a cross-section surface of the sample defined by the x-y plane; milling the sample in the z-dimension to expose a series of cross-sectioned surfaces each defined by the x-y plane, where an x dimension distance between ends of the first and second branches exposed in each cross-sectioned surface changes; calculating a change in distance in the z-dimension between each successive pair cross-sectioned surfaces based on a change in the x dimension distance between ends of the first and second branches between each successive pair of cross-sectioned surfaces and the predetermined angle of the first and second branches; adjusting the milling rate of the focused ion beam after each successive pair of cross-sectioned surfaces if the change in distance in the z-dimension is unequal to a preset distance.