The way it works is easy to understand, but difficult to calculate. I'll give an example:

You post an article where you've selected 50/50 as your payout preference, and at the end of the 7 day period, it's sitting at $40 in value.

75% of the post's value is given to the author, while the remaining 25% is shared among those who curated, so your end result is $30 to you and $10 to curators.

That $30 is then split in half due to the 50/50 payout preference, so you'll receive $15 of it in SBD. The remaining $15 you will receive as Steem Power, and that amount is based on Steem's value at the time of payout. If Steem is worth $5 at payout, then you would receive a total of 3 Steem power.

Your final reward total would be thus be:
15.000 SBD and 3.000 Steem Power.

If you did a 100% Power Up on your post option, you'd receive everything in Steem Power. In that case, you'd get $30 (75% of your post's value) / $5 (Steem price) = 6.000 Steem Power, or 2x what you'd earn at a 50/50 payout.

I hope this helps! :)