I've been using a spreadsheet to calculate my Battle Arena perks for several months now. I use it to balance out my attack, health, and stamina, to make sure I keep the maximum attack perk. For the entire time I have used it, in order to get to the next tier of bonus, for example 50-100%, you actually needed 51.00% in order to move into that tier. After last Friday, it doesn't appear to be that way anymore, as I am sitting at 50-100% attack perk with 50.79% in attack. Not sure if anyone else has noticed this before. I have confirmed with other accounts though.
If I remember correctly there's been some minor issues with the BA perk calculation. I'm in the process of checking with the developer on that. Updated: So it looks like there was a small error with perk calculation but it should have been fixed up as of today's Arena. Please let us know if that is not the case!