I would suggest something like 3lb per gallon i.e. 15lb made up to 5 gallons with water.
Now all the suggestions thus far have forgotten a few things, that to achieve a sweet mead with the White Labs yeast (and whoever it was that mentioned the Wyeast version is wrong, thats a different yeast with a much lower tolerance), you'd need to either mix the batch for a higher gravity or start lower and then when it gets to something like how you'd want it to be, to cold crash it, then hit it with stabilising chems (sulphite and sorbate).
The White labs sweet mead yeast, has, from memory, a tolerance of 15% ABV. To get that, you'd need a drop of 110/111 gravity points (110 equates to about 14.94% ABV, and 111 to about 15.08% ABV). So if you mixed it to something like 1.118 (presuming 1.000 is taken as dry/finished) you'd be left with about 1% worth of residual sweetness.
It's fair to point out that it's far easier to ferment dry, then to back sweeten to your desired level, than it is to start higher on the gravity front, so that it should finish with some residual sugars.....
The choice is yours but I'd suspect that if you got 15lb of the honey, you wouldn't be far wrong.....