Calculating exactly how much K-Metabisulphite to add in order to stabilize your mead can be a bit tricky, since it depends on the pH of the mead and also on the ingredients in the recipe. So the most accurate way to add sulphite is to measure the free SO2 level with a sulphite test kit, before and after a K-Meta addition. However, you don't need to be that precise because there is a pretty wide spread between the level at which sulphites become effective, and the level at which you can start to smell or taste the chemical in your mead.
Sulfite will exist in several different chemical forms within your mead, and the amount in the form actually available for anti-microbial or antioxidant activity will depend not only on pH, but also on the amount of chemicals that will "bind" the sulphite. But, in general to stabilize a mead you're shooting for a "free" SO2 level somewhere between about 30 ppm and 50 ppm. Most people cannot detect sulphites in a wine or mead until the level gets up above 300 ppm.
You can use a "rule of thumb" (which is included in the NewBee Guide chapter on bottling) that says 1/8 teaspoon (one gram) of K-Meta powder will add 150 ppm of free sulphite to a US gallon of mead (assuming it is at an average pH value of 3.5). So, let's say you have 5 gallons of mead (that's approximately 19 liters). To add about 50 ppm of free sulphite (assuming none is bound up by other compounds in the mead) in your 5 gallon batch, that would be a little less than 1/4 teaspoon (less than 2 grams). Since some of that may be bound and you can't predict how much, and some of it may be in a form that is ineffective if your pH is not right at 3.5, then it is usually a safe bet to add a little more. So, 2 grams, or 1/4 tsp of the powder, is a good addition for 5 gallons. Notice that triarchy added about 3 times this amount, but since the total addition was only in the neighborhood of 150 ppm, which is below the sensory threshold, the mead taste wasn't affected by that amount of overdose.
BTW - triarchy is right; you should be sure that fermentation is completely stopped (or at least mostly stopped) before adding the sulphite dose since sulphites won't normally halt an active fermentation. Many commercial wine yeasts have a sulphite tolerance up above 100 ppm; a few can tolerate levels in excess of 325 ppm.
You should add sorbate only if you want residual sweetness in your mead - there is really no reason for a sorbate addition to a dry mead. Sorbate works by preventing yeast cells from budding, so it will act to prevent a new colony from developing in a sweet mead in the off chance that a few cells wake up from dormancy, or that a few wild yeast cell spores make their way into your mead during bottling.
You should add sorbate shortly after the sulphite addition. Sulphite levels naturally are reduced over time, since SO2 can be slowly bound by some chemicals in the mead, and also the gas eventually comes out of solution from your mead and dissipates in the air. Corks aren't a 100% completely hermetic (airtight) seal, so this happens even in bottled mead, although it happens much slower once your mead has been bottled. I usually add sulphite a few hours to a day (maximum) before bottling, and then add the sorbate just before I rack my mead into the bottles. If you don't have a high enough sulphite level in your mead when you bottle and you add sorbate, and if some airborne malolactic bacteria get into the mead during bottling, those MLF bacteria will metabolize the sorbate and create a compound called geraniol as a byproduct. Geraniol smells to some folks like rotten geraniums, and in high enough concentrations it can smell like a citronella candle - not something that I want in my meads. There's no way to neutralize the geraniol once it has formed, so sorbate additions without sulphites can create a potential for permanent spoilage of your mead. If you use sorbate, then use sulphites, too.
BTW - There's a more in-depth discussion of sulphites in this thread, if you're interested:
http://www.gotmead.com/forum/showthread.php?t=14664 You'll note that in there I reveal that I use less than the rule of thumb would suggest. However for a beginning meadmaker, until you get a feel for what will work with your recipes, I'd recommend sticking with the rule of thumb, or testing for sulphite levels directly.
