we use the total virtual Steem supply when calculating the price SBDs (Steem Backed Dollars) start printing. This is because SBDs are designed to be a stablecoin, meaning that they should maintain a value of $1 USD. In order to achieve this stability, the Steemit platform uses a system called the SBD debt ratio to determine when SBDs should be printed or burned.

The SBD debt ratio is calculated by dividing the total virtual Steem supply by the total amount of SBD in circulation. When the SBD debt ratio is above 10%, new SBDs are printed and distributed to users as rewards. When the ratio is below 10%, SBDs are burned, reducing the amount of SBD in circulation and maintaining the stability of the coin.

It's important to note that the total virtual Steem supply is different from the total Steem supply, as it takes into account Steem that is locked up in Steem Power (SP) and cannot be traded on the open market. This means that the total virtual Steem supply is a more accurate measure of the available supply of Steem that can be used to support the SBD price.

In summary, when calculating the price at which SBDs start printing, the Steemit platform uses the total virtual Steem supply.