Currently,

  "virtual_supply": "212716237.198 STEEM",
  "current_supply": "199335134.831 STEEM",
  "current_sbd_supply": "1886630.621 SBD",
  "median_sbd_price": {
    "base": "0.162 SBD",
    "quote": "1.149 STEEM"
  },

So the percentage = 1 - 199335134.831 / 212716237.198 = 6.3%
Actually, 1886630.621 * (1.149 / 0.162) + 199335134.831 = 212716237.198
If calculate with the price I fed last time

  "sbd_exchange_rate": {
    "base": "0.133 SBD",
    "quote": "1.136 STEEM"
  },

then virtual supply should be 1886630.621 * (1.136 / 0.133) + 199335134.831 = 215,449,513.669 ,
the percentage = 1 - 199335134.831 / 215,449,513.669 = 7.48%