Rye Sourdough - Crust Problems

First time posting here - had a question about developing a thin, crispy crust on sourdough. Currently, I have a very nice crumb but a soft crust. 

Recipe: Tartine baking method using 850g central milling bakers craft (similar to AP) + 150g stoneground rye. Baking at 450 in cast iron combo cooker for 20 mins with top on, 22 minutes with top off. 

Is the crust a result of my baking schedule? or is it related to how I'm developing the dough? both maybe?