Well, I don't understand exactly why, but the relation appears to be 65536. So if the network difficulty is 0.063387, that times 65536 is 4154, so a share has to be greater than 4154 to be a block. Or, from a freshly restarted cgminer, the "best share" has to exceed 4154 to have found a block.

With current Bitcoin difficulty of 2,621,404,453.0646, a miner needs a best share of 1.718e14 to find a block, which would probably be represented as 172T (because it's 172 tera-somethings. Terradifficulties?).