Bob, I think there is no need to confuse him with the things from HoH:MC when he is having trouble with the core rulebook. ^^
Andrew, as ErikT said, you are confusing a few things. Let's unravel your example.
(EDIT: just saw your second comment, it seems about it (I'm just unsire if magnetite should be base metal or hard stone). I'll keep the example I gave below, since I've already typed. =P)
Suppose you want two effetcs:
- transforms you in a horse;
- increase your size once you have become a horse.
Effect 1 is simmilar to Shape of the Woodland Prowler (a level 25 effect). Effect 2 is Beast of Outlandish Size (a level 15 effect).
You can make two items, but you realize that you can create a single belt and invest it with both effects. This would require you to enchant the belt as an invested device. Assuming you only need to cast each once per day, you need an item that can receive 40 levels of effects.
The first step is to create the item. Looking at the table on pg. 97, a belt is a small item (x2) and leather has a base value of 2. Therefore, you need 4 pawns to open the belt for enchantment. You can do this in 1 season of work. This already gives you exactly the 40 levels you will need (10 for each pawn), but you begin to worry that once per day won't be enough. You are feeling wasteful, so you give each effect unlimited uses, increasing the level of each by 10. Now you need the item to be opened with at least 6 pawns.
You decide you can use a small magnetite buckle (I think it is hard stone instead of base metal), so it would be 4 (hard stone) x2 (small) = 8. You will need 8 pawns to open, and a Magic Theory of at least 4. But you open it as a compound item so that you can also use the Shape and Material bonuses of the belt.
If you are a verditius and made the belt you can reduce the number of pawns used to open. In your example (craft 3), you wold need 5 pawns.
Now you need to enchant the effects.
You decide to make it a fine belt using horse leather (animal hide has +7 to turn into the appropriate animal). When enchanting it your lab total will be Mu + (smaller between Co and An) + Int + aura + MT + S&M. If you are a verditius and made the item you add your relevant craft (in your example, 3).
Horse leather gives +7, magnetite gives +3 for animal, for a total of 10. With verditius runes you can add your Philosophiae score (in your example, 4). However, the bonus is capped by your MT, so it will be only 7.
After enchanting the first effect you start to enchant the second. It is a MuAn effect. The +7 bonus from horse leather isn't applicable anymore (it's not an effect to change into a horse), but the bonus from the magnetite is.
EDIT2: Looking back now, I can't help but think this belt also needs an effect to change itself into a saddle, linked to your transformation through a linked trigger. Put those 20 extra levels that are still available to proper use.