I don't think we can demand a certain amount until steemit implements the communities feature.
I think 25% of the SBD half of your rewards is a fair number but if you feel to give more that's up to you.
Please note that how much we upvote your post has NO relation to how much you donate, our upvotes reflect the quality and reflectivity of your post.
I think it is fair too, thank you for the clarification Mark.