Thanks again to all who took the time to respond. My conclusions are as follows. Not everyone was enamored with the idea of coming up with a "Base Weight" to begin with. Some found it a nebulous term that lacked clear definition. Others seemed to have a range of base weights depending on circumstances. Still others seemed indifferent about the idea of coming up with an exact number. In any case here is what I came up with when including my own "Base Weight" to the list.

Range: 12 to 20 pounds
Average: 15.53
Standard Deviation: 2.82

I wondered what the average BW was as a percentage of body weight. The average American male is now 191 pounds. I took off 10% because backpackers are more fit than the average person. So: 15.53 pounds divided by 171.9 = 9.0% of body weight. Not bad considering people used to say you can carry 20% of body weight in comfort.
Probably not a scientific poll, but something to think about and more importantly, something to do when it is raining and I can't get out hiking. BTW the curve attached looks like the numbers are normally distributed. I don't know whether this should give me more confidence in the numbers or not.