I'd say:
- A storm going on: base guideline, as this is perfectly natural to have lighting then.
- Grey to dark clouds without a storm: +1 magnitude (this is "slightly unnatural")
- Little to no clouds: + 2 magnitudes ("very unnatural")
- From your hands, in a cave or building: +4 magnitudes ("wholly divorced from its natural context")
There's something to be said about range, but as I'm working from serf's parma, I'm not sure at all. I'd say maybe you'd need to create these in the sky (sight) and redirect them to the earth through Rego, so I'm not sure that'd be more efficient.
Of course, you could probably, with rego and range sight, try to "seize" existing lightning and make it fall on someone, but good luck with the finesse on that.