I'd like to suggest an improvement to the text in the bounty help page, in the section on offering successive bounties on the same question:
Additionally, if you offer multiple bounties on the same question, the minimum spend doubles with each subsequent bounty (50 reputation on the first bounty, 100 reputation on the second, 200 on the third, and so on).
What this doesn't say is that the minimum spend on a second bounty is really double the actual value of the previous bounty. I discovered this to my detriment when I wanted to place a follow-up bounty to reward another answer. The original bounty was for 250. The only bounty it would let me place after that was 500. I was planning to place a 200 reputation reward bounty but that was not possible. If the most deserving answer got 250, it doesn't feel right award 500 to an answer that's also good and valuable but required less effort or provides less value.
To clarify, I don't take issue with this practice as it will only happen rarely and there's a good reason for the doubling, see: "Abusing" bounties for unlimited profit
But even on that thread announcing the change, they talk of minimum spend and no mention is made of the fact that the minimum spend becomes double the actual spend of the previous bounty. Assuming that this isn't a bug, the help text should be updated to clarify this.