I guess if you're using PUR, where the glue thickness is set, you could simply calculate volume (LxHxD). With EVA, it's not going to be that easy. There are also some variables which need to be taken into consideration:
1. Are you side-glueing?
2. Spine thickness
3. Spine length
4. Notching depth
for costing purposes, you could probably work on a worst-case scenario. I doubt it'll make much difference on the glue cost per book.