I'm American and it put me at 66% British...

Edit: and now 99% British every time. Methinks the site is broken.

I'm also an American and get consistently 80-90% British. It seems to think I pronounce "Bob" in an especially British way. I pronounce it in a way that sounds like "Bahb" to me, same sound as in not and cot (I don't have the cot-caught merger). I believe it's the vowel <ä> in IPA that I use. To my ears that's not the way most UK speakers would pronounce it, which I think is more <ɔ> or <ɒ> for all of Bob/not/cot.

I was able to get 90% in either direction at will.

American: over-pronounce your Rs and try to get the æ sound wherever "ah" is called for. (I guess this is a great lakes accent)

British: imitate the intonation of BBC broadcasts I occasionally hear syndicated on npr stations.

Heh. In my regular accent (American) I got 90% American (although, I got 99% American when I spoke normally and didn't purposefully enunciate). But my God-awful British accent got 98% British.

It picked up on the parts of the generic British accent (London accent?) that us Americans think about and exaggerate when mimicking a British accent—tall vowels, especially the "O"s and "A"s.

My normal accent is British but I affected a crappy American accent and got 98% first try. I think this will classify you as American if you just put in rhotic Rs and ham up the vowel sounds.

Here's the trick - the "r" is not as important as that ghastly "o" sound that becomes like an "aaaaaaaa" in General 'Murrican.