This is correct. The 540 height line is usually different from the 540 thickness line because of the variation of surface pressures. But if the surface pressure is exactly 1000 mb, then the 540 height is the same as the 540 thickness line, where thickness is defined as the height between the 1000mb and 500mb pressure surfaces. Usually, though, sfc pressure is a little higher than 1000 mb, so you have this extra air under the 1000-500mb layer, so the total height from surface to 500mb would