Is this a good leveling tool?

If you have a suspended table think twice about using an 8" level. It'll imbalance the suspension and give a mis-read.

Try these instead, in mirrored positions on the platter.
They are so light they won't affect the platter.

http://www.russandrews.com/product-Bubble-Level-Twin-Pack-5021.htm

You can verify the product accuracy by reversing them through 180 degrees at the same location. Just ensure the platter surface is scrupulously clean.

Coincidently, I was looking at the subject of which level yesterday, on the Linn Forum site. The (jist of the)answer was to use a long level(maybe 8-inches or more)on the plinth, because wood has enough minor imperfections that make smaller levels inaccurate. Of course, there are a lot of other things involved in the setup of this turntable, so maybe those make the longer level more accurate for this turntable.

There's no need to level the plinth on the OP's TT. And on a Clearaudio, it's probably not made of wood anyway. You first level the shelf, then put the TT on it and level it at the platter. That's all you need to do.

"08-16-15: Viridian
Well, in some cases, the above is bad advice. Pink Triangle, and others made intentionally dished platters on some models to flatten pinch warped LPs when used with a supplied clamp.

In these instances a laser level adjusted to the inner and outer edges, or the spindle level, would seem to be the best ways to accomplish this."

Those platters are not very common. But if you ever have to level one, the spindle level would be a last choice. You have no way checking anything beyond the spindle itself. So if there is any type of imperfection further out, you would have know way of knowing.

I guess you could use a laser level, but the dish design platter has been out long before laser levels were available. The way you level those is almost exactly the same way you do a regular platter. You use a longer level and make sure it passes over the outside edges (the highest point), in 2 places. And you'll need to take measurements from 2 non parallel lines, just like in the first example.

LOL it's just a piece of plexi-glass attached to $3 pocket bubble level.
I believe that it carries the purpose of convenience and nothing more.
If you adjust the position of your plinth placing bubble level closer to the edges, you can get more precise alignment than if it placed at the very center.