Try this:-
On your North Atlantic Plotting Chart 27th.
Draw a straight line (great circle as it's a Lamberts) from
Tiree N56:30 W007 to Oystr N54 W055
Now measure the INITIAL GC at Tiree - should get 284
Now measure the FINAL GC at Oyster (might have to extend line)
should get 245
Maths average = 245 + 284 /2 = 264
Now measure the AVERAGE GC exactly half-way along track you should get 264. So your rhumb line/constant direction/average GC between Tiree & Oystr is 264.
Please PM me for any further assistance.