You can tell how far along the females are by looking at the arteries that are in the wall of the egg sack. As the eggs progress in ripeness they turn a deeper more pronounced yellow instead of a lighter paler yellow and the arteries are larger and you can see more blood in them. I never looked that close but I think theres one main blood arterie that feeds the sack, with a few smaller ones. Right befor they spawn the eggs are a rich deep yellow with a real pronounced red arterie and have grown to atleast 3 too 4 times the size of the paler smaller eggs of just a few weeks earlier. You can tell when thier ready to spawn by how much blood is in the arterie and how deep yellow the eggs are, they look really ripe and ready to be laid.