The amount of SBD that can be converted cannot exceed 10% of the total STEEM supply (based on the witness median price feed of the last 3.5 days). At the moment the SBD supply equals about 30 million steem.

That's not exactly how it works. When the supply of SBD hits 10% of the internal Market Cap (MC), the amount of Steem converted per 1 SBD gets capped at the rate where the total SBD = 10% MC and no more. However, every new conversion as well as all newly created Steem continues to alter what that amount is. This is why we saw the price feed initially get 'stuck' at 0.42 when SBD went above 10% last time, but then slowly tick down all the way to 0.33 during that time period. The lower that price feed value, the more Steem the remaining SBD will convert for.