That book says 99% because Scharffen Berger makes a 99% chocolate. The equal to that is 100% unsweetened chocolate or chocolate liquor...Almost every store will carry that...Ghirardelli puts one out and so does Bakers chocolate. Those are two most popular in grocery stores.
You can substitute other chocolates for that(like you were talking about). If you substitute the 70% cacao content chocolate for the 99% then I would just say to bring down the sugar 1/4 cup....I would say to try that first and see how it comes out...it will be close if nothing else...but I think it will work fine...it is going to come down to the sweetness level. I do not think that the frosting will be affected negatively in texture or form because of it...
What is the process to make the frosting?
Have a good one,
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