I don't have an exact calculation for that unfortunately. But I know it's relative to the other people who have voted on the same post. The people with the highest SP who have voted on a post will earn the most curation rewards. So the only recommendation I can make is to keep powering up whenever you can.
I'll try and see if I can't find a tool (or information) that helps you determine your curation rewards.. but I'm not if it exists.