Spell power tends to lie on a smooth gradient, but sorcerers usually use a classification system that splits them into three simple spell tiers: Low, medium and high.
As a general rule, the more powerful a spell, the more fleshrunes you’ll need to cast it. In theory this limits the amount of power a spell can have. In practise, if you have sufficiently compatible pawns… or clones…
High
A spell is considered high tier if it uses seven or more runes.
High-tier spells can be powerful, hard-hitting monstrosities:
- Blacken the sun for three consecutive days.
- Cleave a continent.
- Power an impenetrable magical shield around a city for several days.2)
Certain high-tier spells can theoretically ‘break’ the whole system, e.g. someone might successfully uplift themselves into perfect spellcasting, erasing the downsides of spellcasting (well, minus the pain, at least initially, until they alter themselves not to feel pain) and possibly even speeding up the process (e.g. increasing their own speed and dexterity to superhuman levels). The only real guards against this are that high-tier spellcasting is really fucking hard to begin with and, much like nukes on Earth, there are cultural norms against proliferating the associated knowledge and capabilities.
Which doesn’t mean everyone adheres to those norms, of course. Insert the setting-equivalent of the Hzataalar Kaea here, basically.
Medium
A spell is considered medium tier if it uses four, five or six runes.
Medium-tier make up the majority of spellcasting, generally considered as having a sweet-spot between risk and pay-off. This is usually where you handsomely pay a professional sorcerer to do something that e.g. would take your culture maybe a decade of menial work to achieve (maybe significantly diverting a river to a more convenient spot), or e.g. does useful alchemical things such as “make lots of fuel for this space ship we have from this lake of water”.
Practically all alchemy sorcerers use in practise, be it mass-preserving or otherwise, is medium-tier on the high end, usually requiring five to six runes.
Note that it's usually comparatively easy to turn a medium-tier alchemy spell into a high-tier alchemy spell, as the structure of spells “instantly turn the entire core of this planet to gold” is remarkably similar to “slowly turn the substance under the runes to gold until you run out of substance or I tell you to stop”, about equivalent to adding a suffix to a word, although care must be taken to pick the right suffix.
Low
A spell is considered low tier if it uses one, two or three runes.
They are usually not considered worth the risk of botching them (although single-rune spells usually just have the risk of misspelling the rune and doing something completely different). But in theory you can e.g. light a fire with spellcasting (one rune), or carry around an orb of artificial light (two runes), or commune with an animal (three runes, although it sure will be a very weird mental experience, since animals do not think like you at all).