Interesting problem CS, The North Atlantic Plotting Chart we use is a LCCP as well with SP's of 5140N and 7820N. The Chart coverage is from 3500N to 7500N. The size of the circles will depend on the latitude, but for the life of me I cannot remember the calculation for the scale change. I'm sure there is someone out there who does. I use AutoCAD Map incidentally, but try to avoid circles like the plague for the reasons stated above - ie I'm lazy!
Regards
TOD