I have also been told less than 10% once a h/b is detected. Until then the chances are much higher at around 20 to 25%.
I had an u/s done at 6 weeks with this baby as well and the bpm was 113. I started freaking out because I heard it was suppose to be around 160!
I studies this on the net for a whole day and discovered that at 6 weeks anywhere around the 100 to 115 mark or so is healthy and exactly where it should be. If it's below 80 then the chance of miscarriage is much higher, but not definite.
I am sorry to hear they have only detected one h/b so far. Maybe the other one was just in a funny position. Let us know how that goes...