3" CCVM - How much alcohol is in it

[gsugg]

I've had a 30 gallon copper pot I built and a 2" x 36" CCVM for some time. I recently bit the bullet and put together a 3" x 48" stainless modular CCVM for an upgrade. My question is how do you calculate how much alcohol does it take to fill a 3" x 48" column filled with lava rock. I know it's a needle in a haystack question, just looking for an estimate. Thanks guys.