So, Incantation of Lightning is (in 4th) a 7th magnitude spell. It requires narrow specialization for a magus to start play with it, and can still take a while to become "casual" to cast in play. To my mind, it seems it should be easier to Rego lightning from an existing storm that to Creo it from clear air, but 4th core has no such spell.
Looking at the guidelines in 4th, I think Re Au 20 "conduct occurring weather" (however it's exactly phrased) is a fitting guideline. Agree or disagree?
IoL is base level 5, but has a pretty big modifier for creating lightning out of nothing: +4 magnitudes.
So it would make sense that creating more "natural" lightning could be done with lower magnitudes effects.
Sadly, we now enter the domain of YMMV.
For exemple, some troupes will feel that conjuring lightning out of a storm to strike with pinpoint accuracy will be slightly unusal (+1 mag), others will think it is, nonetheless, very unnatural (+2), while others yet will require you to target the clouds (sight range) to create the lightning.
I had thought about this issue before, but I'm quite rusty, sorry.
FWIW, IMO, the Range is the range to the target. So you don't need Sight to fry Suicidus the Fool.
As a example of what you could do, I'd range several spells depending on the clouds:
Actually, it's not about "out of nothing", but it's about "originating at ground level, rather than in the upper air, and possibly indoors/underground". In practice, I think the modifiers were retrofitted to ensure that IoL, a legacy spell, stayed at its "classic" 7th magnitude.
In principle, if you can see the sky, you should be able to call down a lghtning strike at R:Sight ("pulling it" from the upper air, where it naturally belongs) with a Level 25 spell: Base 5, +4 R:Sight, no other modifier because Lightning from the sky is not even slightly unnatural. This is true even during a clear day, when it may be unusual, but not unheard of (hence the expression "Bolt out of the Blue"). We know today that these come "bending" from a nearby thundercloud, but it was not known in the middle ages.
Oh wait, 4th edition. That's quite different!
Yes, that guideline is what I'd work with if directing down naturally occurring lghtning from an existing storm. From my experience: roll a stress die, a lightning bolt bolt occurs on a roll of 20+ if there are serious clouds ahead. Add +4 to the roll if it's raining, +8 if it's an actual storm, +12 if it's a violent thunderstorm (in the last case, you'd get lightning on average every 3-4 rounds, about once every 20 seconds). I'd probably add +1 magnitude for "great precision" if you attempted to target anything smaller than a building.
So... eyeballing, from Clouds of the Thunderous Might, I'd say: if you have clouds overhead (it gets better if you are in a thunderstorm):
I would not use the guideline to call down lightning "at will" from an existing cloud or thunderstorm. That's higher level. I'd rank that at least as high as "cause clouds to produce rain" (Level 30), possibly 1 magnitude higher for a phenomenon of great strength (though probably not, eyeballing it all). So I'd add about 10 levels to the quantities above.
This means that you get a discount compared to Incantation of Lightning (or a more serious one if you are willing to forego precision) and you also get better range and relative incospicousness, but you need to be outside with appropriate weather. Awesome for a magus Cursed to always find bad weather wherever he may go.