Mass-Produced Reagents

Lets first look what ArM5 really says (bolding mine again):

So, first a magus needs to know the Reagent or Theriac he is attempting to create. And second, for a bad or average Finesse roll when magically creating Alchemical Steel (A&A p.76) or a Tonic of Gold (A&A p.77), you get badly carburized medieval steel and some tonic with a mix of the right ingredients that might well kill you - but both will still be the right sort of thing.

Now to the ArM5 p.153 ritual Conjuring the Mystic Tower, a hold-over spell from ArM4 p.227. To roughly fit with ArM5, it got 3 magnitudes for elaborate design. I am not aware of precise rules to determine the Finesse rolls for creating objects that were never made by craftsmen, like a tower made from a single block of stone.
I suppose that, unless the Finesse roll is botched, dimensions, shape and design of the chambers are assured by the extra magnitudes, as are level floors, a roof keeping rain water from seeping into the tower, and roughly working chimneys for the labs. At a bad or average Finesse roll, everything beyond is left to the troupe: I recall medieval stairs that require spontaneous magic to climb safely. And don't expect a tower of Chartres cathedral, unless you really rolled well.

HoH:TL 101f ritual Twinning the Tome got 5 magnitudes for intricacy, and an Intellego requisite adding one more magnitude to boot, to roughly fit with ArM p.77 on Creo, or the later HoH:S p.60.
A more refined method to provide the same result with lots of achievable Finesse rolls of Ease Factor 9 to 15, and which does not skirt the basic guidelines by just adding lots of magnitudes, is TME p.101ff The Superb Scrinium. I certainly would equip the HoH:TL Milvi rather with one of these than with the older Twinning the Tome and lots of vis.

A rule generalizing the addition of lots of magnitudes to a spell, in order to lower the Ease Factor of a Finesse roll, is something to discuss in your troupe, not something I'd expect from the still outstanding ArM5 books. AFAICS these added magnitudes are an expedient to quickly introduce specific Hermetic effects, without going through designs like The Superb Scrinium. But perhaps somebody wishes to derive the set-up of a Superb Building Site from HP p.29ff The Great Tower?


EDIT: The Ease Factor for the Int+Finesse roll to make a Tonic of Gold by Rego or Creo craft magic is also just 14.

I am Ok with the conclussion, adding extra magnitudes can low the ease of the Int+Fin roll.

There are a few problems with the interaction of craft magic- finesse and spellcasting which sound great in theory but simply were never addressed or apparently considered:

  1. time use- if I conjure the mystic tower, aside from it being made from a single stone (which could in theory be done with mundane means if an appropriate stone were located), is that the sheer size of the project (paid for with orders of magnitude) would drastically increase the amount of time it would take a mason to construct such a tower- thereby increasing the finesse requirement for reasons that have nothing to do with finesse.)
  2. ignoring actual craft ability- while it may not be necessary to know how to perform the craft yourself, it should be helpful
  3. innate complexity and source materials- copying a book, for example, would normally require a scribe ability over a season (or more depending on the book and the scribe) with a given scribe skill. at the same time it is hard (especially with something like an intelligo requisite) to see what a failed finesse roll would result in when the spell really requires no artistry or interpretation, it simply copies something that exists.

I would use than the most easy way is the Leadership use.