How do you figure? Before block 209,999, there were 10,499,950 BTC in existence. Then block 209,999 added 50 more, bringing the total up to 10,500,000 BTC. That was an increase of 0.000476193%. To calculate the annualized inflation rate, you take the natural log of 1.00000476193, multiply by the number of blocks in a year (52,595.9856), and exponentiate the product, base e, which yields 1.28461321188. So the annualized inflation rate at block 209,999 was 28.46%.