The Ritual Spell system was designed before the Player's Companion Spell Building system. I had already made a decision, in running the Auran Empire Campaign, that some magic was simply too powerful to permit as simple spells if I wanted historical versimilitude and economic rationality for the setting.
Is it possible to reverse-engineer the modifiers for ritual spells? Probably not directly. In creating ritual spells, I have tended to allow small effects to scale up to an area of effect that's huge - much more rapidly than otherwise would be permitted. Hence Harvest and Cataclysm, which are very weak as single target effects, can affect an entire 24-mile hex with their effects. The largest area of effect of a non-ritual spell is probably Growth of Plants (3,000 square feet) so this is many orders of magnitude greater area of effect! But there are no ritual spells which can do even two orders of magnitude more damage to one target (100d6 per level of the caster).
So there's a basic asymmetry there! I would say if you're designing a spell for a very large area of effect, go with the following:
1. Design a spell with an area of effect of up to 3,000 cubic feet (or square feet if height is irrelevant), or single target.
2a. If it has an area of effect, add "ritual spell with area of effect increased by 4,646,400 times" as a modifier. That will increase 3,000 cubic feet to 500 square miles. For Blast spells it is a x2.5 modifier. For Transmogrification spells it is a x24 modifier.
-> to do a 6-mile hex, make the modifier x1.375 for Blast and x6 for Transmogrifcation maybe?
2b. If it has a single creature target, add "ritual spell targets up to 60,000 creatures within 500 square mile area" as a x5 modifier.
Harvest: Harvest increases the Land Value in a 500-square mile area by 2gp per peasant family per month for 12 months. Typically, selecting one rank of a proficiency will increase an adventurer's wage by 10gp per month . So this is one/fifth of a proficiency. Since granting a proficiency costs 20 points, this effect costs 4 points. 3,000 square feet of unintelligent plants has an area of effect modifier of x0.3. The duration is 12 months, which probably should cost the same as indefinite (x3.5). It's a beneficial effect (x1). Range is 0' (x0.6). Total cost for the spell is 2.5 points. We make it a "ritual spell with area of effect increased by 4,646,400 times", which increases area of effect to 500 miles. This increases the cost to 60 points, making it a 6th level divine ritual.
Cataclysm: Cataclysm kills half of the peasant population in an area of 500 square miles, along with damaging structures. A blast spell dealing 1d6 points of damage to every creature in the area of effect with no saving throw works as the effect, as most humans having 3-4 hp, a little over half will be killed by 1d6 damage. The base effect cost is 27 for 1d6 damage per caster level x 0.1 for 1d cap = 2.7. The range is special - the caster must have seen the domain or met the ruler. Let's call that a x5 modifier, raising the cost to 13.5. Duration is instantaneous. No saving throw is permitted. For the area of effect, a 20' diameter sphere is close to 3,000 cubic feet, so that's x2. Overall the spell has a cost of 27 points. We make it a "ritual spell with area of effect increased by 4,646,400 times", which increases area of effect to 500 miles. This increases the cost by 2.5 to 67.5 points, making it a 7th level divine ritual.
This is just me riffing. Let's play around and see what comes of it.