It's a starting point. It's not even possible to get that angle on certain vehicles, my DEX8/DESC10 being one of them. Now, I always use the outer most hole on the arm (if there's a choice) and then use what is essentially equal up top front to rear. Several years ago, before big bores, I was very limited with standard bore spring selection so moving top and bottom locations to achieve balance was necessary. With the wide variety of spring rates today, there's usually one that fits my desired location.