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I'm looking at the example spells in Thaumatology: Ritual Path Magic and the very first example, Air Jet, seems to have an error in the calculation of casting cost.

I'm aware that SJG sources often use what seems to me a bizarre method of calculating costs; if they add or subtract a percentage, it's a percentage of the original cost, not of the calculated cost up to that point (IOW, 10 - 20% - 40% would be 4, not 4.8 rounded to 5).

Here, we have Greater Control Matter (5), with modifier Damage, External Crushing 3d (+8, though this isn't written in the spell block -- itself with Double Knockback +20%, Jet +0%, No Wounding -50%).

Based on TRPM 39 and Spell Modifiers on TPRM 18, then, I see 5 + 8 (= 13) + 20% (of 13, = 15.6) - 50% (of 13, = 6.5, so total of 15.6 - 6.5 = 9.1 rounds to 10), so x3 for one Greater effect ought to be 30. Am I misunderstanding the calculation method (very possible), or is this just a calculation error that got past the editors?

On further reading, I now see that External Damage is tripled for a given energy cost, but now the error runs the other way -- 1d (which gets tripled to produce the External Crushing 3d) is +0 which is still zero after applying +20% and -50%, so either this is (5 + (0 +20%, -50%) = 5) x 3 = 15, or it's (5 + 0) + 20% - 50% = 3 x 3 = 9.

If it matters, I'm using the PDF v 1-1, purchased and downloaded from Warehouse 23 last night (so it's presumably not an old edition).

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Yes, you seem to have an editing error there. Here's the errata I've just submitted, having figured out what's going on.

There are inconsistencies in the description of Air Jet on p. 39. These are worth fixing because it's the first example in the Grimoire, and uses modifiers in way that aren't fully explained in the main text. I'm working off PDF version 1.1, dated January 2023.

There's an inconsistency between the spell header:

"Spell Effects: Greater Control Matter."

and the "Typical casting" block: "Typical Casting: Greater Create Matter (5)"

This matters, because Control costs 5 and Create costs 6. It is genuinely unclear if Control can do the job, or Create is required.

Then there's complexity over the spell cost. This starts at either 5 or 6, depending if it really uses Control or Create.

Then it adds 3d External Damage, with Double Knockback, +20%; Jet, +0%; No Wounding, -50%. The enhancements and limitations total -30%, which gives no modifier to the energy cost, per "Enhancements" p. 17. The damage is being treated as if it were a missile, per "External (Indirect) Damage" on p. 17, so the caster only pays for 1d damage, which costs 0. Overall, the cost is 5+0 or 6+0, tripled for the Greater effect.

So it looks as if the actual errata are:

"Spell Effects: Greater Control Matter."

should be:

"Spell Effects: Greater Create Matter."

and:

"Greater Create Matter (5) + Damage, External Crushing 3d (Double Knockback, +20%; Jet, +0%; No Wounding, -50%) (1)"

should be:

"Greater Create Matter (6) + Damage, External Crushing 3d (Double Knockback, +20%; Jet, +0%; No Wounding, -50%) (0)"

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    \$\begingroup\$ Okay, there's a conflict between the top of the block, which lists Greater Control Matter, and the casting detail, which has Greater Create Matter -- but then there's an error in the costing in the casting detail, since Create is 6 but External Crushing 3d ought to be 0 (not 1, as parenthesized in the casting description). \$\endgroup\$
    – Zeiss Ikon
    Sep 4, 2023 at 12:11
  • \$\begingroup\$ In other words, they did it wrong and got the right answer... \$\endgroup\$
    – Zeiss Ikon
    Sep 4, 2023 at 12:13
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    \$\begingroup\$ @ZeissIkon: figured it out, submitted the errata. \$\endgroup\$ Sep 4, 2023 at 12:47
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    \$\begingroup\$ As my understanding grows, I had reached the same conclusion. I've been putting off learning RPM for years, but now I need it -- so it's good to know I'm finding the same costs others do. BTW, they might also want to make clear that they're using Limitations and Enhancements as per Basic and/or Powers along with what's explicitly in the RPM book. \$\endgroup\$
    – Zeiss Ikon
    Sep 4, 2023 at 18:01

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