If all of your model terms are orthogonal, then you can indeed determine the amount of variance accounted for by that model term. PROC GLM will do this. If all of your model terms are NOT orthogonal, then there is no unique decomposition of amount of variance accounted for by that model term. This can also be seen in PROC GLM by examining Type I, Type II and Type III sum of squares (and of course, there are many other decompositions of the variance). In other words, in this situation, you cannot come up with an answer to the question.
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