I had done some research for my heather Ale and while I do not know the IBU (I am pretty sure there is no such value), 2 ounces was recommended in the boil and 2 dry myrtling.
When I made my heather ale, I used Saaz hops to bitter for predictability and heather to enhance the bittering and then late in the boil for aroma and flavor.
I would be cautious dry myrtling as you will almost certainly introduce wild yeast and bacteria.