Quick HR Question

Any rule of thumb for Lactate Threshold with regard to average heart rate over a half marathon?  I ran a 1:48 half yesterday and averaged 180 bpm pretty consistently (see race link for mile by mile breakdown: http://www.beginnertriathlete.com/discussion/forums/thread-view.asp?tid=136740&posts=2&start=1)

Just wondering if any of the experts can gleen anything from my data.  I'm guessing that my LT must be around 185?