Technically speaking the debt limit never goes above 10%. At the moment we have 42 million Hive that supports the value of HBD. This number doesn't show anywhere but it can be calculated by dividing the Hive supply by 9.

If the virtual supply exceeds the maximum supply (which is equal to the hive supply + hive supply/9) then the latter is used for conversions (the so-called haircut rule). In the theory, the blockchain can support a circulating HBD supply of 21 million at the current feed price of 0.5.

The formula to estimate the "haircut" price is circulating HBD supply/(hive supply/9). So, if the witnesses' median feed price is lower than the "haircut" price then the blockchain uses the latter for conversions.

For example, if the HBD in circulation was 30 million instead of 9 million then the conversion value of HBD to hive would be 0.70 (21/30 or max circulating HBD/circulating HBD). This formula only applies when the debt level has reached its limit.