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My post that earned $20 roughly ended up being something like $17 after the 24 hours waiting period, wouldn't final payout decrease a lot if there was a 7 days wait ?

User expectations are extremely important. That should be fixed.

I think throwing the author and curation reward together on first view was a mistake. In the name of KISS it should be the authors reward that you see first, then when clicking to see the details you see the curation rewards added.

There is a GitHub issue open for it. It is going to be a complicated change to calculate the curation vs. author rewards in realtime though (on every active post). It is something that will need to be implemented at the blockchain level in order for it to be done efficiently.

https://github.com/steemit/steemit.com/issues/87

Agreed. And create the nice big pot for curation instead of the comments!

No the payout won't necessarily decrease based on increasing the payout window. It applies equally to all posts. On the margin there is no change. Price fluctuations may work in either direction (likewise vote changes).