It's the recent_claims variable that's a couple of digits too big. Perhaps I'm using a less than optimal equation to work out voting value. Although, having quick look at your link, you seem to be using recent_claims as well, without problems I'm assuming.

I ended up creating an algorithm to get a pretty good estimate of the value, but I've just noticed it's putting out wildly too big vote values for people with large sp. I might need to revisit it, or try and work out how you use recent_claims without getting a NaN error. Are you using a big number library?

edit: I've just noticed that when I run the app on my pc I get less wild results than when I run in as an Android app on my phone. Bloody strange..

edit2: I've just checked my calculation, and it gives allegedly accurate results for high steem power too. So there must be something wrong with algorithm I found to calculate vote value. Does this sound right?:
proportion = vests/recent_claims
100% upvote at 100% power = proportion * reward_balance

That's giving me different results than @penguinpablo's vote calculator.

edit infinity: Got it working. Woot!