You sir, are incorrect. It works by kbps (kilobits per second) despite the framerate.
Out of boredom, I've decided to test your theory. All the tests use the same video clip (ripped from dvd, fairly complex) with VP61 at 480x352 with a bitrate set to 300kbps. Audio doesn't matter, but its 32kbps AAC.
So I had it encode at 30fps, 15fps, and for shits and giggles: 5fps. By your formula the results should be 332 for the 30fps, 182 for 15fps, and 82 for 5fps.
I encoded them all then got the average bitrates, which turned out to be:
So as you see, the entered bitrate works by seconds, not frames.
PS: Don't take this post as hostile or rude, I'm just extremely bored and it makes me grumpy