Assuming you have read the whitepaper. All the formulas for how post rewards work are explained there.

Regarding the 50/50 payout, let's say you have a post that is paying out $10 to the author (after curation rewards are removed). $5 of that will be as SP. The other $5 will be as "liquid payout".

The liquid payout portion is normally SBD, but if the debt ratio is above 2% - then it starts to pay out a portion as STEEM instead of SBD.

If the price of STEEM were twice as high - you would be getting a $20 payout, with $10 SP and $10 liquid. The $10 SP is the same amount of SP as the $5 payout - the SP is just worth twice as much. As far as the liquid portion - if it were being paid as SBD you would get twice as much SBD ($10 worth). If it were being paid as STEEM - it would be the same amount of STEEM, just worth twice as much.