By 1 percent Cocoa Butter, I assume you mean to add an amount of Cocoa Butter equal to the entire batch of chocolate, seed and all? For instance:
900 grams chocolate (including seed) + 9 grams of cocoa butter?
The tip regarding the lowering of the set point is very useful and will be tried if the state of the tempered Milk Chocolate begins to lose its luster with the addition of Cocoa Butter.
Correct on 900g + 9g of cocoa butter as approximation of 1%.
If you find the change of viscosity too much you may need to go to 0.5%.
You need to add the cocoa butter to hot chocolate and mix for 10-15 minutes.