A post payout is determined on the basis of proof-of-brain in Steem blockchain where the users of the community determine the post value in a decentralized way by exercising their Steem Power(staked STEEM).

The Author reward and the Curator reward share the post payout in a ratio of 50/50. So the Author gets 50% of the post payout and the Curators get 50% of the post payout. The 50% of the author part is further divided into two parts in which 50% of author reward(means 25% of the total payout) is paid in liquid form and the remaining 50% of the author reward(means 25% of the total payout) is paid is staked form(means SP).

The liquid payout is generally in SBD, but that is dependent upon the Debt ratio in Steem. So the debt ratio decides whether the liquid payout will be in SBD or STEEM or SBD & STEEM both.

SBD is considered as a debt in Steem blockchain and it has an upper cap of 10% of the market cap of STEEM. So whenever the market cap of SBD exceeds the 10% cap, the system stops printing new SBD coin. Right now the market cap of SBD is more than 10% of the market cap of STEEM. That is why the post payout is in STEEM and SP which was earlier SBD and SP.

The following rules apply for the post payout-

If the SBD market cap is less than 9% of the entire STEEM market cap, the blockchain prints 100% SBD. So the post payout comes in SBD and SP.

If the SBD market cap is more than 10% of the entire STEEM market cap, the blockchain stops printing SBD entirely. So the post payout comes in STEEM & SP.(which is exactly the case now)

If the SBD market cap is between 9% and 10%, the blockchain prints SBD according to this formula: 100% x ( 10 - debt percentage). So in such a case the post payout comes in STEEM, SBD and SP.

Debt ratio= Coin marketcap of SBD/Coin marketcap of STEEM

Analysis

The debt ratio generally goes up(more than 10%) whenever the Steem price falls. Similarly, it goes down when Steem price rises. So it is inversely proportional to the price of STEEM in general. But this relation holds good with the assumption that 1 SBD= 1 USD. If SBD rises substantially & disproportionately more than the rise in STEEM price, the same relation may not hold good.

Now let's plot a chart on this relation by assuming 1 SBD= 1 USD

The current price of STEEM= 0.12 USD.

Take a 0.02 USD interval of STEEM price in increasing order.

So let's take intervals like 0.15 0.17, 0.19, 0.21, 0.23..........

Assume that the current supply of SBD and STEEM for this projection(Even though new STEEM coins are minted daily, considering the huge supply in millions, those new supplies are not taken into consideration in this analysis).

Total supply of STEEM= 352,701,927

Total supply of SBD= 7,361,844

STEEM price(now)= 0.127 USD

SBD price(now)= 0.77

Debt ratio(now)= (Total supply of SBD x SBD price(now))/(Total supply of STEEM x STEEM price(now))

Debt ratio(now)= (7361844 x 0.77)/(352701927 x 0.127)

Debt ratio(now)= 0.1265= 12.65%

Now let' calculate the Debt ratio when STEEM will be at 0.15 USD, 0.17 USD, 0.19 USD, 0.21 USD............

I have made a calculation in the excel sheet and also drawn a chart based on that data. Let's have a look at the chart.

Assumption made in this analysis

(1) 1 SBD= 1 USD

(2) Total Supply of STEEM is assumed to remain fixed for this projection.

From the above analysis, it can be roughly estimated that-

When the price of STEEM will trade above 0.21 USD & below 0.23 USD, the post payout will be in STEEM, SBD, SP.

When the price of STEEM will trade above 0.23 USD, the post payout will be in SBD & SP.

As long as the price of STEEM is below 0.21 USD, the post payout may continue to be in STEEM & SP.

The actual co-relation of Debt ratio with price of STEEM may slightly vary as the total supply is getting added with new STEEM on a daily basis from the reward pool(with its inflationary model of reward pool).

The inforgraphics, graphs, analysis are my original works.