As The Incantation of Lightning is a dramatic, well known, and flavorsome spell in the setting it makes sense that a defense might be created. Personal wards are not cumbersome to create but they have to be specific, so a paranoid wizard in Ars Magica needs to create a range of them and activate them proactively.
Personal Ward Against Lightning
Rego Auram 20, R: Personal, D: Sun, T: Individual
The caster is warded against all natural lightning, and magical lighting effects must be at least double the level of this spell to affect the target.
(Base 10, +2 Sun)
As a core guideline the Base 10 to allow the Target to be protected against a type of very severe weather, such as lightning.
What is odd in this spell is to allow magically generated lighting to also be warded. The intent is to have a defense against Incantation of Lightning and a rego wards is appropriate way to do that. As a very specific ward I think it is reasonable to allow a lot of protection.
Circular Ward Against Lightning
Rego Auram 25, R: Touch, D: Ring, T: Circle
The targets within the warded circle are protected against all natural lightning, and magical lighting effects must be at least double the level of this spell to affect the target.
(Base 10, +1 Touch, +2 Ring, +0 Circle)
This and over 390 other new spells for Ars Magica can be found here.