I kinda touched on this question in the become a dragon tread...Say I want to give my parrot the ability to breath fire and a magic might.
It would be muto animal base 25 +1 for the fire requisite +1 for the vim requisite. Where do i go from there? How much fire can it now breath, or how much might does it have?
Does the orginial base of 25 apply to the ignum chart? (which is pretty darn good) Or does the +1 give me a base 5 in ignum effects, and I add magnitudes to increase the effect?
Same question applies for granting might, how much does the +1 for the requiste give, the base 25, or 5 points for the +1 and I again add magnitudes to get more might?
As I understand requisites to work, you need to determine what the equivalent magnitude of the additional effect would be if it were a stand-alone spell. For instance, if the parrot came breath the equivalent of Pillum of Fire (which I believe is CrIg 15), then it would add +1 Mag. If your parrot's fire-breathing is the equivalent of a CrIg 30 effect, then it would add atleast +2 magnitudes for being sixth magnitude or higher.
You would calculate the Vim effect in the same manner. I don't know how much Might a given magnitude of CrVi should provide, but whatever it is, if the base effect is five magnitudes or less, add +1 magnitude, if it is 6 magnitudes or more, add +2 magnitudes.
The guidelines for Animae Magic (Arcadian Mystery) are +1 level per point of Might, with a minimum of one on top of the base spell which would otherwise grant no Might. However, I have strong doubts as to whether granting Might is within the reach of vanilla Hermetic Magic.