The way I would tackle hand-making your own contour lines would be to just lay down color blobs, lowest areas in black then proceed up through the grays each on its own layer say in increments of 10, set those layers to zero fill and then put a layer style of stroke on it. That way you can get complete control of where your lines will be. It might take a while and lots of trial and effort to get them right but it gives you complete control.