Before 30 minutes, split between author and curator grows linearly with time. Bullets in FAQ are examples. That is, if one upvotes at 1 min 37 sec after posting, it's not zero reward, it's actually 96/1800 = 5.3% multiplier.
Another thing to consider here: is this multiplier for what? Before or after 30 minute mark, curation reward depends on how popular the post was before you upvoted. The lower post reward was at the time of your upvote, the higher your reward will be (other things being equal, namely total post reward at the time of payout).
This creates an interesting mind game for curators to play. To maximize profits, you want to upvote before others because of the latter mechanic, yet not too early because of the former.
Thank you very much @teo-nyx for your feedback and explaining this with a formula. I believed it worked that way, but was not sure of exactly how.
Yes, this can be a complicated game to play. Interaction is more important than rewards I think, and just upvoting after 30 minutes or even on the last day is fine for me. I won't hesitate to upvote a post that deserves it at any time.