I always found it easier to just put chunks of the drug in question on the scale until i got the desired weight. Crystal molly (or any other kind of drug) is far more marketable than powder (if you're dealing, your customers will assume that powder is less pure - especially if they know you get it as crystal, because then, why would you powder it except so you could cut it without detection?)
Crushing hard crystals while minimizing mechanical losses is not as easy as you'd like it to be. I've always used several layers of strong paper, plastic bags tear through and your precious drugs start leaking out after a few blows.
I'd just leave them as crystals unless I really had to make powder for some reason