The devs math is a touch off as it will be closer to 200M coins through pow. The next halving will occur very soon so that means ~75% of the minable coins have been mined so far. I believe it takes roughly 20 days from mining until the coins are able to stake so the first pos sgould come next weekend. I can't check the code right now to confirm so I'm going from memory but I'll check when I'm home later. And the 20% is the amount a staked coin will earn I believe in the first year.