If you use DME to replace all the pale ale malt, you won't really be "mashing" the crystal, you'll be "steeping" it. That's fine, of course, but good to know the terminology.
If you want to do a partial mash, keep some of the pale ale malt in your recipe. Figure out how much grain you can mash, subtract that from the bill and make up the difference with DME. The 0.6 conversion factor is a good route, but if you want to be in more control of your OG, you can measure the gravity post-mash and adjust the amount of DME up or down to hit your target. Not essential, though, and the more DME you plan to use to begin with, the less important this is.
Regarding your recipe, without some roasted barley or roasted malt in there, you're not going to get the typical roastiness that stouts require. This isn't necessary for making good beer, but if you're interested in brewing something that meets the traditional definition of the stout, you may want to look into that.