Check the bottom where you are, too.
You can't make your block too heavy. Blocks weigh just half of their above-water weight when submerged. If your float is moored at a nominally windward shore, you should add still more weight.
Winnipesaukee's lee shores tend to be sandy or, in recent years, muddy. (Due the oversized-boat wakes of the past decade). Soft bottoms will "hold" the block better.
Rocky bottoms offer a poor surface. In a strong wind (like last Saturday's 52-MPH-in-gusts -- thanks Don), your float could repeatedly jerk the block across a rocky bottom into deep water -- and end up floating off somebody else's shore!
Use more chain than you think it needs. If the chain or shackles are left submerged over winter, inspect the steel parts every third season. These waters eat steel. (And aren't too friendly to stainless, either).
If you elect to prepare a poured concrete block, I'd suggest molding a hole through it, or using an oversized stainless steel eye bolt. (I use a huge stainless plate which penetrates the block, so it is exposed for the shackle attachment on either side of the block).
My community-lakefront neighbors on the windward shore use 4-by-4-by-4 granite blocks for their middle-sized boats. At first I thought that was overkill.
Maybe not.
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