Bit rate only really matters if you're shooting something with a lot of movement or complicated fine detail. I pretty much always use 200 unless it's a talking head scene. That said I rarely shoot at 400. For me I'd rather always use 200 since external drive storage is so cheap these days. I likely could get by with 100 but I also would be very irritated if I shot something and then found artifacts in the footage because the bit rate was too low.