A cinder block won't sink a pallet, it will probably take at least three. You have to put a lot of weight on it or it might just kind off suspend just under the water, it's happend to me.

I don't use them anymore, they are way too heavy.

It's a great fish attractor if you have the muscle and a big enough boat and help.

As for how much weight to sink a quantity of brush, you learn as you do it. But, as a rule of thumb I put two saplings about 1" to 1 1/2" diamater per cinder block.

I carry an extra block or two or some concrete or rocks with wire already on them when I sink stuff in case I get it wrong. Then I just tie the extra block on and let her go.

Good luck