New blade to cure a wobble

There are bushings at Lowes that a pennies apiece; and there are bushings from machinists that cost real money. I suspect the latter have much closer specs. That said, I would try for a bushing (we are talking about the disk in the center of the blade) would best be as thick or thicker than the body of the blade. Anything less (depending on how tight the bushing fit on the arbor) might allow it to twist. Being a bit thicker should not be an issue because we almost always use a concave flange washer that would accommodate a thicker bushing. The purpose of the flange washer and yes it must be flat on its perimeter area is to hold the blade straight on the arbor (reduce flexing and any twisting on the bushing arbor axis. I've only bushed and installed a few blades but thats my simplistic thinking on the factors involved. A bad bushing and or a bad flange washer can defeat any good blade. I think the bushing that I used on my 12" MK 225 blade to get it down to 3/4 cost on the order of $13 for two. Several years later, the blade is still running and I consider my $13 well spent.

let's be sure we are using the same terminology. The bushing is a flat washer like disc that has the inside diameter of the arbor shaft. The outside diameter is the same as the hole in the blade. Bushings are to my knowledge always flat. The flange washers are the discs that are placed on each side of the blade to steady it and keep it true. The hole in the flange washer is an exact fit for the arbor shaft. The flange washer is almost always concave so that the outer margins is in true contact with the blade body, the center area is not necessarily in contact (I think this insures that the widest area is in contact and flat. Normally the flange washer has to be of a certain size depending on the size of the blade. A large blade with small flange washers would be easy to distort and one with a larger sized flange washer would be sturdier. A bushing that does not exactly fit the arbor or the hole in the blade could twist or tilt slightly which could allow the blade to shift slightly. Only you can decide what your problem might have been and how to rectify it. I hope this off the top of my head view is of value.
Lapidary shops can sell good stuff and can sell not so good stuff. The fit is the key.

I think the bushing is just to center the blade until you are able to tighten the nut that compresses the flanges. The flanges are ultimately what hold the blade in place, not the bushing. Assuming that the bushing allows you to properly center the blade on the arbor, the thickness of the bushing shouldn't matter once you tighten the flanges.

Nevertheless, I agree with buying some quality bushings and not using generic hardware store washers. I use high quality stainless steel bushings that are machined to something like +/- 0.0001" of their nominal size. They create a very snug fit both within the blade arbor hole and as they pass over the shaft, and I have been using the same ones for a long time. Here are some littles ones I bought recently to reduce 5/8" arbor to 1/2" for my trim saw.

Check out Ebay.  I have gotten quality ones like the ones in the photo I posted for about $5 each, I think.  That sounds like a lot of money for what basically look like washers, but they're not washers.  They are high quality stainless steel and the dimensions are very precise.  Assuming the blade arbor hole is drilled to spec, the fit perfectly.  It takes a little effort to get them into the blade, but they fit perfectly and don't move as the blade is being slid over the arbor and the nut is being tightened.    

MK Diamond blades seem to have very precisely drilled arbor holes.  A lot of Chinese blades do not.  

There's no reason you can't get very clean cuts with a 20" blade.  I've seen cuts with a 24" blade that looked like a 500 grit finish, with absolutely no visible saw marks.