Making a 30' Sanding Dish

I know that most people say you don't need a sanding dish, but I can see the advantages of having one since it can be used for both truing up the sides and bracing the top/back. So I thought I'd give it a try for the 30 foot radius dish used for the top. After thinking about it for a few days, I came up with what I thought was the quickest and easiest way to do it. I didn't want to have to make any special jigs, I thought I could route it free hand. I calculated the depth based on the formula for a circle: x**2 + y**2 = r**2, solve for x given r = 30*12 and y being the radius from the center of the dish. I then printed this out in 1/4" increments.
First step was to draw concentric radii of the 2 foot diameter disk in 1 inch increments. Then route out each ring to the appropriate depth with a 3/8" router bit. The trick though is to not route the WHOLE ring, but to leave a small sliver each time so the router base has the original surface to ride on for subsequent rings.
Once this was done, I came back and routed out the slivers and then used a chisel to take out any rough spots. Since this board is MDF, a chisel goes through it like butter. Then I got my $20 random orbit sander (from Harbor Freight) and began to sand.
At first, it didn't seem like I was getting anywhere. But all of a sudden, the lines just started to smooth out and disappear.
It was easier and turned out better than I anticipated, the whole thing took about 2.5 hours. Now I can get back to working on the sides.
A 30' dish has a max depth of 0.200", the difference in depth between any of the rings was only a few mils on the inner part, and grew to about 1/32" over the last inch. Therefore 1" increments worked fine. But this difference will double when I make the 15' dish for the back. I probably will want to space the rings closer together on the outer part of the 15' dish to lessen the sanding required.