White on white should be no harder than any other exposure. You need only place the tonal value accurately on the histogram. I Spot meter off the white highlight adding 2 to 2.7 stops above the metered value depending on how flat the light is, with my Canon's. Try plus 2 with your D3s I would spot off the white highlight adding 2 stops, if it clips come back a bit, too dark add a little. Some Nikons (D300) meter different than other cameras, and require less exposure (1.3 stops above the metered value). Once you have your camera zeroed in rendering white should always be very close to the same value. Most problems come about when using large meter patterns (Eval/Matrix) with the subject size being smaller and of different tonality than the background. A Spot meter renders whatever is in the small pattern as a mid-tone value, and negates the background influence. You need only figure out how much or little light to add or subtract from the meter recommendation to render the tone as desired. Example black -2, white + 2, it becomes easy once you get the values memorized.
Meter Patterns determine Exposure, Priority Modes change Variables.