Umm . . . a very rough guestimate would be to shoot for a SG a tad above 1.100, but not too high else the fermentation should stall out. Take a look at the potential alcohol tables at the bottom of this page listed here:
http://winemaking.jackkeller.net/hydrom.asp
Lalvin 71B-1122 has an alcohol tolerance of 12 to 14% so you'd want to go a bit higher than the potential alcohol levels listed in the table, for that range of potential alcohol.
You can assume honey, in general, will contribute approx. 35 points per gallon, and that's for honey dissolved in water to make up a must of one gallon. So, for a starting gravity of 1.100, you're looking at about 2.8 lb.s of honey per gallon to get you there. Maybe about three lb.s of honey per gallon will get you the mead you're looking for, 12 to 14% alcohol but with sufficient residual honey to make it slightly sweet.
Of course, all this depends on what translates into "slightly sweet" according to your taste. Any idea of what that might be for a final gravity?
All this is some very rough guestimation on my part. Others may have more specific figures.