Purple Shaft lash question.

i have a .590 Purple Shaft in a 340, with Aries domed pistons, and open W2's. Lash has been set at .25/.25 cold and I'm wondering how far I can push it (.18/.20 cold?). What are you setting your lash at with these cams and does anyone have info as to why Mopar wants them set up so loose? Thanks for any help!