How do general guidelines work?

Hi me and my friends are stuck up on the rules regarding General guideline spell effects, basically is the spell effect only changed by the Base effect or does it also get recalculated after adjusting for Range/Duration/Target? For Example:

PeVi Demon's Eternal Oblivion (ArM 5 Core p 160) is a spell that reduces a demon's might by an amount "equal to the spell's level" now this simply says level, so is this the level of the base effect or the level after Range/Duration/Target?

The part that's really confusing me is this:

In RoP Divine under Wonder's guideline (p 56) the second general guideline lists the following: "create a natural substance that harms non-divine supernatural creatures, such as water that corrodes like acid or light that burns like fire, doing + (BASE LEVEL +5) damage"

Why does that one specific base level of the effect? Is that just an inconsistency in the wording or are we meant to be adding the total level to the power of the spell?

Residual question: Can a Holy Magi use long term fatigue for a longevity ritual?

Generally speaking you choose a base level.
Say 20.
You read the description of the guideline. In the PeVi example it is base +2 magnitudes iirc. You then chose parameteres. Voice, which is +2 magnitudes.
Your spell is level 30, and destroys 30 might points.
So for this example spell, the "destroying" power is equal to the level, because of the parameter.

Should have the parameter of the canon spell been sight, your spell would have said that its destroying power is "level of this spell -5".

For your RoPD example it is this: chose a base. Say 15. This creates a substance doing +20 damage. Now you need parameters, durations, etc.

As for the long term fatigue, no. See RoPD p69, second column end of the first paragraph: "Magi may use this special vis when ceremonially casting spells, but not with other kinds of magic or laboratory work — actual vis must be spent to enchant a magic item or study the Hermetic Arts." Casting the longevity ritual is not a spell because you don't have any casting total.