Yaarel
He-Mage
The DMs Guide (page 283) distinguishes between one-target damage and multi-target damage. The original post of this thread does too, while noting the official spells are more like a 7 to 9 ratio. Despite inconsistent spells, the ability to choose better spells and avoid less good spells makes multi damage roughly 78% of singular damage.
Regarding the area of Fireball versus the line of Lightningbolt, each has their use. The area is best to first-strike a cluster of targets before engaging them. But once combat is in progress, the line is better to hit a target without damaging an ally.
In a separate part of the DMs Guide relating to Creature Size (249), there is a discussion about how many targets one can expect in a spell effect. Of interest here:
• Circle: Targets ≈ radius/5
• Line: Targets ≈ length/30
Thereby, Fireball with a 20-foot radius equates to about 4 targets. But Lightningbolt with a 100-foot line equates to about 3.33 targets. The Fireball assessment feels right. In my experience, maybe Lightningbolt is more like 2.5 targets, but if rounding the number to 3, that sounds good enough to me. The benefit of Lightningbolt is avoiding friendlyfire. So when I use it or see it used, it is mostly to assist a distant ally by taking out a boss, and if one or two mooks get in the way of the Lightning, that helps too.
In the Players Handbook, the spells at a slot level differ wildly in effectiveness. They ignore the advice in the DMs Guide. Humorously, the DMs Guide says: "If a spell is so good that a caster would want to use it all the time, it might be too powerful for its level." But then despite the sound advice, the designers intentionally made Fireball too powerful for its slot level.
By the way, it is more than spell selection and caster-versus-noncaster that needs spells to balance better. Everything in the game engine relies on spells. At low levels, different kinds of features are easier to compare and balance, and there is more familiarity with them to discern their desirability. But high level features are more difficult to assess, being less familiar, more abstract, and often shifting to a different kind of game, such as conditions becoming more threatening relative to hit point loss. When designers try to assess high level features, they comparing them to the spells at that level. So when the spells are wonky, everything gets wonky.
Most damage spells assume a save for half damage. That is what the original post lists for each slot. According to the DMs Guide advice, "if your spell doesnt deal damage on a successful save, increase the damage by 25 percent." So, at slot 5, a spell like Cone of Cold deals damage that corresponds to about 35 damage, and save for half. But if a spell at that slot is all or nothing, it should deal about a fourth more, roughly 44 damage. Spells that are rays like Disintegrate and spells that roll an attack tend to be all or nothing, and should deliver more damage when successful.
There is enough good advice to balance spells reasonably well at each level. Start with the damage spells, because the math is more obvious. But then start comparing other spells to the damage spells by feel. Which spell would one rather have? Eventually, the one gets a strong sense of what the nondamage effects are worth too. For example, one can seriate the mobility spells like Fly and Teleportation from worst to best, lay them across the slot levels, and get a clear sense of what mobility effects are worthwhile at each slot. One knows one is accurate enough when choosing between a mobility spell and a damage spell is a genuinely tough choice.
