The question is interesting and I think your method is fairly valid, what I'm wondering is, what difference does it make. What information is it going to give you that will change anything about how you deal with firewood?
I think it's a darn good way to estimate the amount of solid wood in a cord. Increasing the sample size would probably tighten up the estimate but it's a good method. Not sure how this changes how we deal with firewood, but there's something to be said in knowledge for knowledge's sake.
Well said Mr. Williams! and welcome to the forum if I missed you before. I think any prudent firewood hoarder (consumers in the forum like us who don't process) will know what to look for when dealing with a retailer.
