I'll have a goal peak mileage that I'll want to hit then Badger mile it. I use the newer "Jerry" system, where you base volume on 7 min miles for men and 8 min miles for women, not based on average training pace (which was borne out of necessity without GPS watches).
I just think of it as, if you're trying to use volume as a training stimulus, it makes more sense to me from a scientific standpoint for it to be judged in terms of time stressing the system rather than miles covered. So when you're more fit, whether it be month to month or over the year, your training volume stays the same but distance covered increases or fluctuates.
So for example, I'll set the goal at 65 miles @ 7 min pace = 7:35 hrs. And I'll try to stay within 90% of that, going lower during certain periods of training. But early on, I might only be able to handle a weekly average of 8 min pace, which means I'm only covering about 55 miles a week. As I get more fit, I approach the 65 mile a week goal, but my volume hasn't changed.