Based on my limited testing so far, that website is accurate for a post that is already worth about $10. If an article is worth less than that, your vote will be worth less than that website shows, and vice versa.

Minnows can have a significant impact in a couple of ways:

• Votes from a handful of minnows can get a post to appear in the "hot" category; a place where whales often look for quality content. Get your post to appear in that category and it is far more likely to get a whale upvote.
• Dolphin and whale bots look for minnows who consistently upvote popular content before it becomes popular. Bots will then follow these minnows' every moves and upvote content after the minnow upvotes it.