There's a few factors to consider.......
1, what honey did you use for the initial ferment, a strong tasting one or a lighter more mild tasting one.
2, what kind of ferment did you run/manage ? Was it a lower gravity/alcohol one or a higher gravity/rocket fuel type one ?
3, what do you want from the finished mead ? Sweet, dry, medium or just showing a lot of honey characteristics from the original honey flavour ?
That's 3 points, there's actually quite a number of points worthy of consideration.
What I do is just run the ferment "as is", I don't particularly manage it for high or low alcohol etc, I have 2 main yeasts I like to use, lalvins D21 or K1V-1116 - both will hit 16% reasonably easily. I've been trying to lay off extra acid and tannins as I can add those later, too taste.
So once the ferment has finished, I now let it settle and produce a sediment. Then rack it off. I then take a reading to see where the gravity is, then I back sweeten it to something like 1.010 - this is done now, because one of the things that you can get when back sweetening with honey is a haze i.e. you've done everything else, like got it racked, cleared and pretty much ready to bottle, just need to back sweeten, so you do that with honey or honey syrup and it causes a haze in the mead. I think it's a protein related haze.
My recent efforts that have done that, I left bulk ageing, in the hope that they'd clear naturally, and those that didn't (most of them) had to have 2 part finings to get them clear. I did ask around the forums for suggestions and the result was to use either bentonite or sparkoloid, but while I can get the former, the later doesn't seem to be available here (and shipping would be a killer). Plus I couldn't find time to get to the HBS so I just used the 2 part finings that I had available - they worked well.
So that's why I tend to back sweeten it at that stage, because the other solids help to remove the back sweetening haze when they are dropping out of suspension into lees......
The type of honey you actually use ? well that's up to you, but don't forget, the first and most prominent taste of a mead will be the honey that has been used for back sweetening, especially if you find that you have to use a lot i.e. for high alcohol "rocket fuel" type brews, to remove (mask) the medicinal/mouth wash taste.
So if you wanted some of the original must honey taste to come through, then something lighter/less strongly tasting would be the suggestion, if you're not fussed then you can just get away with store bought cheaper blended stuff, or if you wanted more of the taste of the back sweetening honey, then a stronger (and maybe darker coloured - as that's also affected by this process) tasting honey.
The suggestion that buckwheat is a "cure all" above isn't quite right, as the buckwheat that I've managed to get isn't as dark as some might suggest. It would seem, that, as it's of Polish origin, it's more like the lighter "Western Buckwheat" than the darker "Eastern Buckwheat" that's available in the US. Or what the hell, something really funky, like avocado or even some imported "Killer Bee" honey.....
Seriously though, I'd suggest that you use the same honey as was used for the original must, as you'd probably have a little left over or can make that the case when you buy the initial amount, so that there will be enough left over for back sweetening with.
Oh, and I can't stress enough. There's no real way of working out exactly how much you'd need, but I allow for 1/2lb per imperial gallon (4.55 litres), but mix it up in very small quantities and sweeten to hydrometer numbers, otherwise you'll end up like I did the first time and get something that's amazingly over sweetened. There's some info about the number is you search the forums, but I just (as previously mentioned) sweeten to about 1.010 a.k.a. "medium sweet".
Dunno if any of that helps or not......
regards
fatbloke