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There are several points that possible to improve

If you are working at least with c# 2.0 (as I expect because you mentioned Nullable<decimal>), you could replace ArrayList with List<Ingredient> or even IEnumerable<Ingredient>. This will make it more intuitive, and later you could remove type casting from SumPercentages

foreach (Ingredient ig in f.Ingredients)
{
    SumOfPercentages += ig.Percentage;
}

Then, if you are working at least with c# 3.0 you could, rewrite SumPercentages using LINQ

public void SumPercentages(Formula f)
{
    f.SumPercentages = f.Ingredients.Select(ig => ig.Percentage).Sum();
}

Last point, to implement ICloneable for both Formula and Ingredient. Then replace

this.Ingredients = _Ingredients;

with

this.Ingredients = Ingredients.Select(ig => ig.Clone()).ToList();

So Ingredients will be copy of initial array. This will prevent situation when you unwittingly change list of ingridients after you have created Formula.