This is from Curt Stock's "Melomels for Dummies" document, that is required reading for fruit meads, but worth the read regardless. Here's his approach to SNA:
SNA was developed by the commercial wine industry as a way of supplying nutrients as the yeast needs it during the growth phase – kind of a just-in-time delivery. Healthy yeast are essential for a clean fermentation with less chance of off-flavors or the production of higher alcohols (fusels) which can give mead a burning sensation on the back of the throat – the “rocket fuel” sensation.
I prefer to use Fermaid-K (yeast energizer) and diammonium phosphate or DAP (yeast nutrient) for adding the additional nutrient requirements of the yeast during fermentation. One teaspoon of Fermaid-K and two teaspoons DAP should be adequate for a 5 gallon batch. You can mix them together for a stock blend and add them using the following schedule:
Add ¾ teaspoon yeast energizer/nutrient mix immediately after pitching yeast.
Add ¾ teaspoon yeast energizer/nutrient mix 24 hours after fermentation begins.
Add ¾ teaspoon yeast energizer/nutrient mix 48 hours after fermentation begins.
Add ¾ teaspoon yeast energizer/nutrient mix after 30% of the sugar has been depleted.
Personally, I just add the first addition to the must when I mix it with water, then add the remaining ones at days 1, 2, and 3. His amounts work in melomels, but if this is just a straight mead with no fruit, I'd probably bump these additions up a bit (maybe a full teaspoon for each addition). The fruit in melomels does provide some additional nutrient that you won't be getting in a straight honey mead.
Fermaid K works great, but you could use another yeast nutrient if that's what you have. I've used the Wyeast nutrient that I use for beer in place of the Fermaid with no ill effect.