(these are my own words though). The subjective preference and intention of the capitalist has no bearing on the value of the product being manufactured. For example, say that the the capitalist wants to manufacture spindles. It is of no consequence to the value of the spindle (qua spindle) if the Capitalist wishes to manufacture them using gold, since the socially necessary material is simply iron (or what have you) to make a perfectly functional spindle. This is a way to think of social necessity in terms of qualitative factors. In terms of quantitative factors (which is far more important, I think), what determines the social necessity of the quantity of labor time for a particular object X is the quantity of labor-time necessary given the mean degree of skill and technology of the particular society. If, using the mean degree of skill and technology, the labor-time necessary to make object X is 10 units, then that is what determines the value of object X rather than, for example, object X being produced at a small shop with 25 units of labor. This assumes, moreover, that the use-values of both are the same (even in the case of spindles - for obviously if the use value differs, then object X which took 10 units will not be the same as object X1 that took 25 units, and object X1's value would be determined by the socially necessary labor time thereof).
So, if someone objects that the gold spindle will be worth more than the iron spindle even if it took the same amount of labor time this is a conceptual confusion on two fronts: (1) they differ in value because the gold spindle has a different use-value than the iron spingle, i.e. we are now comparing the use value of gold with the use-value of the spindle; (2) in gold we would need to determine the socially necessary labor-time in that process of manufacturing, and this is what would account for the difference in value. But in this example, as I mentioned, I thinking of the use-value of the gold spindle qua spindle, where the gold has no bearing on that object's value as a spindle.
I tried to be as non-gibberish as I could. I would love for other to add to this.