Thank you both for the replies.
I read as many THH trip reports I could find and have, through an incredibly unscientific method, deduced that 300mg is probably an okay starting point.
My first batch will be 1g spice/1g plant material/300mg harmine/300mg THH. I'll report back. Thanks again!
EDIT: FWIW, I'll be enhancing Mullein, Marshmallow Leaf, Damiana, Lavender, Chamomile, Skullcap, Blue Lotus, Hops, California Poppy Leaf, Wild Dagga.