8 ounces (weight) of DME happens to equal out to about a cup (volume), but it has nothing at all to do with the coincidental fact that 8 FLUID oz (volume) also equals a cup (volume).
a good rule of thumb is 1/2 cup DME to 1 quart water
so mix up your 1 cup DME and 2 quarts (liters, whatever) water, boil it for a couple minutes, cool it, and pitch your yeast. this will work fine.
getting a decent scale makes everything easier.