Tim Capps wrote on Aug 26^th, 2010 at 5:12pm:

It is a known bug that FSX will route you from New York to London via the scenic western Canadian detour.  Co Pilot from Abacus will create a correct route, and so will FS Build.  But for default FSX planning, I use your method of creating a GPS route, then dragging the line to the waypoints.  It is tedious, but can result in a more realistic flight plan.  I put in a lot more than a couple, though.

When doing a transcontinental flight i usually will look at Vataware,
you can look for flights that have been flown there and it give you
a route.  There is also 'vroute'.

Once I have a route string, I'll use FSBuild to actually generate
the flight plan. Still requires manual inspection for any errors
but  I'm fairly happy with the results.