Yeah, I use D&D spell-lists as a rough gauge what a spell or similar spell should cost. Mana costs roughly equal the Level of the Spell. Mostly, it's a case by case basis thing. I have the Mage /Ars Magica spheres in the back of my head when thinking about it. Also, thinking asking myself 'Is this a Minor, Medium or Major effect?', 'Can I offset the benefits some how?' or 'What are the long and short term outcomes of the spell?' Also: give them what they work for not what they hope for.
Fireball is one of those weird spells, though. In every edition that I've played there's this moment that your Wizard hits third level and holy fuck I can nuke shit out of these guys. It's such a big step to go from 2d4 to 3d6 area (plus feats and advances). The rub is that you might only be able to pull that off once during the whole day where the Fighter and the Paladin are able to dish it until their HP runs out. Which incidentally, far later than that of the Wizards.
Generally, thought, it's not going to be a one-to-one thing. Mostly, it's like the Wizard is gonna wanna do something, and I make a call based on what I think is fair and being a fan and all that. It's also salient that my player has very little interest in combat generally, so I don't think it'd even occur to him that he could be casting spells that did that much damage.
The lesser version clause is on there if ever I need to tone down the scope a spell without blocking it too much. Like, if the Wizard wants to do this very powerful thing at third level that's really clearly a ninth level spell, instead of saying 'I'd cost nine points of mana and you only have four' I can say 'it'll cost two points but it'll only do this much of what you want and have these limitations.'