Head retention in a raspberry stout

Had some issues with head retention in my last stout where I added 32oz grade b maple syrup post-fermentation.  Am planning a choc raspberry stout with 4-5lbs of raspberries post-fermentation.  Is it simply a matter of adding some dextrin malt to my mash?  Mash higher to compensate?

For a stout my personal choice would be to add some flaked barley.

How much?  10%?

shouldn’t need that much, a couple oz maybe 4