Ahhh, but these animals are in captivity and we have the ability/responsibility to feed them appropriate sized prey.
The distance between the eyes rule is probably an overly safe one ... most bugs crunch up pretty easily, but it's certainly not a bad idea to give them (maybe a few more) a size of prey that is easily manageable.
The range of sizes of 5 month old veileds is surprisingly large. Therefore (in my opinion) it is not enough to just know the age of the animal. How big is it? Snout to vent?
-Brad