I usually spot meter the part of the sky where I want to see color, meter the shadow where I'd like to see detail, and meter the brightest part of the sky other than the sun itself, and decide what I'm willing to sacrifice. The part of the sky where I want to see some color has to be close to the middle, and then if I'm shooting slide film, there can be a half stop either way that is potentially a "correct" exposure, but might change the emphasis. So I use all that information to decide where and how to bracket. If the horizon and lens I'm using might lend itself to using an ND grad, I add that possibility to the mix.