Well, you're correct about the formula not being accurate because I ignored chains longer than 9. However, the amount of 10-chains is pretty small compared to 9-chains. I did some testing which showed that the amount of 10-chains is only about 2% of the amount of 9-chains. My old formula was essentially this:
blocks/day = chains/day * (1 - fractionalDifficulty)
Correcting for 10-chains that becomes:
blocks/day = chains/day * (1 - fractionalDifficulty) + 0.02 * chains/day
Now that the difficulty is around 9.94, the estimated blocks/day becomes 33% higher.