Your assumptions about line breeding is a good rule of thumb.
It certainly isn\'t a law, as what you do in setting up matings depends on what you are trying to accomplish. If your goal is to \"set\" the slate legs in your blacks it sounds like you are on the right track. It\'s always helpful to write out your goals in advance, take copious notes, and compare results with your original plans. We learn more by making mistakes than by accidentally doing something right the first time, at least that has been true in my case.