I am currently reading GURPS Magic. I want to use Thaumatology ritual path magic and have run into a problem. When using missile spells you have to roll against Innate Attack but how do I determine that value?
If you want to throw Missile Spells you use the Skill Innate Attack with the Missile specialization, likewise there is a Beam specialization for Beam Spells.
You find the Skill Description in the Basic Set.
For spells like Fireball, Lightning, Ice Missile, and Stone Missile, which innate attack to use is explicit. If this isn't the case for your Ritual Path Magic (built like a Power, as I recall), double check with your GM -- though as a GM, I'd rule that it should use the innate attack that makes sense. If the ritual creates a missile, it'd use the "Innate Attack, Missile" skill; if it creates something that acts like a beam, you'd use "Innate Attack, Beam", etc. Note also that these might have an Acc bonus (the ones for the spells I listed do) that applies if you Aim