that is correct, it is the ratios that are evenly spaced in this model. if you wanted to do thicknesses that are evenly spaced, that would be interesting as well, but in this case it's the ratios. The reason that the corresponding thicknesses aren't evenly spaced is that we are *dividing* 20 by these ratios each time, so for example 20/r, 20/(r+d), and 20/(r+2d) aren't evenly spaced, even though the ratios are. I have to admit I had to get out a piece of paper to figure out what was going on here!!