Stephen
Thats certainly a worst case route that would allow you to continue with a low cloud base. However it will add quite a bit of time.
Ive been up to Kirkwall several times and never had to hug the coast quite as much as you are planning.
I'd prepare a couple of other routes which reduce distance & time;
1. Direct (or more of less), say PTH - INS. Perhaps include a few waypoints to roughly follow the A9 and stay clear of the highest group.
2. South of Aberdeen, say Edzell - Banchory - Inch - Nairn. You should have a good idea before you depart Dundee.
If you are stopping at Dundee you should be able to get a good idea of which route to follow before you depart.
Fingers crossed for a successful trip no matter what route you take. Hoping to get back up there myself next month and get a few of the remote islands in the log book.