Consider Demon's Eternal Oblivion (ArM p.160). The guideline is "Reduce target's Might by Level + 10." Let's say I only want to target weak demons.
I pick base level 0, +2 Magnitudes to make it Voice range. As the base is below level 5, those two steps add just one level each. That gives me a spell with Level 2 that reduces Might by 10. That's easy for most magi to spontaneously cast, and get a high enough score to penetrate said Might of 10.
First, I don't think most troupes allow 0 nor negative bases. So let's go with base 1.
Second, you'll want to look at the errata. The guideline is
Reduce a target's Might Score by the level of the spell + 2 magnitudes
So base 1, Voice, would be level 3 and reduce Might by 3 (noting that levels listed in guidelines refer to the guideline level, the level of the spell without increases for anything else). Base 2 would be level 4 and reduce by 4. Base 3 would be level 5 and reduce by 5. Base 4 would be level 10 and reduce by 10.