Hannah - I am by no means an expert in this area but i can tell u about what my neuro did - i just had the biopsy on the 19th and am also still waiting. My neuro took 2 samples one from my upper thigh and one from my ankle (i have also heard of a sample being taken from the calf). My neuro said that the reason for these placements is that these are the areas that are standard and have the most data to compare to for analysis. The fact that you did have only one taken is kind of odd BUT my neuro said that the reason to take 2 is to see if it is a length dependent neuropathy or not. The single sample can still be analyzed for small fiber nerve density however . Where was you biopsy taken from. Also were your samples sent to therapath labs in new york (this is where mine were sent). Also about the idea that if your normal was in 50th -- it is true that they can take a sample from a sight that is expected to have normal density for you however observation of the single site can still show mildly abnormal (less than 10th percentile) results or other problems with the nerves (i think something about swelling of the nerves) both which can yield suspision of SFN and may then yield another biopsy later for conformation.

Hope this helps some! I know what you are going through some what waiting for these results is driving me insane (my symptoms sound very similar to your too)
Hang in there!