A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Sep 29, 20:20 -0400
The USNO website provides the time of transit as 12:51 PM.
My longitude is 72°47'.6. Therefore my delta longitude is 2°12'.4 or 2.21 degrees. At 4 minutes of time per degree, the offset is 8.82 minutes or 8 minutes 49.6 seconds.
12h51m - 8m49.6s is 12h42m10.4s
And you have it exactly correct. So now you know exactly how to calculate transit or meridian crossing for your longitude!
Your current practice of shooting 1/2 hour before to 1/2 hour after may be a bit excessive, but certainly you can't be faulted for being careful! I'll leave it to the cognoscenti to proffer their opinions on this point.
I hope I have this correct. Your LAN for yesterday (September 28) was at 16:42:10.4 GMT.
I've started shooting the Sun about 30 minutes before MP and then waiting for the Sun to descend to the same Hs figures recorded before MP. Using the method mentioned in Bowditch the several results are averaged based on equal Hs and time. The results I'm getting are a few seconds within calculated MP. The clouds being cooperative, of course.