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Bob,

Your 1975 vol. 2 Bowditch should have a table for converting radio bearings from GC to rhumb lines. Attached PDF at 1-7 shows a correction table from GC to Mercator (Rhumb line) that covers differences in longitude up to 7.5°.

http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/RNA/117chapter1.pdf

This may be all you need.  For larger longitude differences you will need to use a 1958 edition Bowditch radio direction conversion table. I don't have the 1958 Bowditch with me so can't tell you what page or table number but I seem to remember that it is the first table.

Greg Rudzinski 

From: Bob Goethe
Date: 2018 Jan 18, 22:47 -0800

Thank you for this, Frank.

Let me post a followup question not just to Frank but to whoever would like to respond:  I can use the Bygrave equations to calculate my initial great circle course and the distance from my current location to my destination.

Is there another equation (or set of equations) that will let me say, "If I start from a certain latitude and longitude, and travel 200 nm on a course of θ°, what will be my latitude and longitude at the end of my journey?"  In a sense, I think my question is "Can I plan ahead my waypoints for a great circle route WITHOUT using a gnomonic chart?"

If a mathematical solution to this is straightforward, that would be great. 

If a table-driven approach is easier, I own a Volume 2 of Bowditch (1975 edition), as well as - of course - a downloaded Bowditch 2017.  If there is a table in one or both of these editions of Bowditch that will answer my question, I lack the skill to recognize that table for what it is.

Thank you all for your assistance to me.

Bob