Steve,
I understand the equation. I have done the calculations myself and you're answer is wrong. If you like, I can point out the error but I thought that you might want to discover it yourself.
It's a simply units mistake (I kept making it too trying to verify this) that makes you're answer 1000 times too low. The Reynolds number in my loops are approx 12000, not 12.8! That makes the flow highly turbulent, not highly laminar. Using the typical flow rates in most residential GEOs I would guess that almost every residential ground loop is flowing turbulent.
Disclaimer: I'm not a geo consultant, nor PE, nor do I claim to be anything more than a interested nerd in this forum, so I could well be totally wrong, but my numbers match the online calculator & also pass the sniff test when comparing to claims/calculations of others about turbulent loop flow in geo thermal fields. I'd be willing to bet a cold beer on them.
Len