rear drum brake adjustment???

Hi there,
In addition to replaceing my lower ball joint (another post) I am also replacing my rear brake shoes and drums on my 91 olds 88. It all seems simple enough, except the manual makes reference to measureing and adjusting the distance between the shoes after I put on the new ones.
This confuses me because I thought the shoes just pushed outward and conformed to the drum, and therefore did not need adjustment. Can anyone help? Thanks in advance.

Unless someone has another way to be more accurate,the way I know is to remove the drum and adjust shoes out slightly,replace the drum and repeat this process untill you have a slight drag.then put two lugs on backwards to hold the drum in place then apply the parking brake and release it and again check for a slight drag.