You basically have to guess the first time, then if you really care enough, measure it during your first batch. Keep in mind though that the difference between mash in water and first runnings volume is a combo of grain absorbtion (which is relatively constant) and deadspace. So, let smith figure the absorbtion and you just subtract that to figure deadspace. Now you know for all future batches. That being said, after two batches I haven't cared enough to measure. I just make up for boil volume by sparging more if necessary.