@saywha I think math doesn't work here much, because paying system is so different which we can't explain by math. And I got confused because of your calculations. You said 40 dollars payment but is it SBD or USD?
They're showing SBD on our posts. For example I got $10.09 from a post and $7.80 came to my account and $2.29 went to curators.
From $7.80, I got almost 3.9 SBD and 0.673 Steem Power. Steem was about 5.5 USD in that moment while SBD was about 6.2 USD. I don't know how they calculate Steem Power that they will give from articles, but only thing I understand from this is: Until Steem is 5.8x of SBD, don't choose 100% Power Up. Wish we had 100% payment by SBD option. And why 5.8x? It's because 3.9/0.673 is almost equal to 5.8.

In your calculations there is something that I don't understand. If you got $40 from a post. $10 will go to curators and $30 will come to you.
If you divide 30 by 2, you will get 15 SBD from that post. If you divide 15 by 5.8, you will get 2.586 Steem Power. My math calculation is like that (I didn't add calculations for currency value, because it's not necessary in this platform until you want to get your money back).

But I agree with you if we take it simple. Because nobody should use 100% power up option. 50/50 is so much better.