substitute for bittersweet chocolate?
You don't need to add much unsweetened. Chocolates vary, but figure the difference between semi and bitter is about 10% cacao solids. So if you have a recipe calling for say 200 grams (7 ounces) of bittersweet at 70%, and you only have semi 60%, the difference in cacao solids in that amount of chocolate would be 20 grams (a little less than 3/4 ounce). Unsweetened is 100%, so you could add 20 grams of that. (Using grams because the math is easier, if you want to convert the recipe, multiply ounces by 28.35). Or, if there is sugar in the recipe, you could just decrease that by the same amount.