Use the calculation tool
here to get an idea of how much honey to use in the must. Just remember to add enough water to GET to the 5 gallon mark (not add 5 gallons of water to the honey amount).
Also, D47 is rated for 14% ABV. Don't assume it will go higher than that.
Read up on how to treat a mead must on the
Got Mead forums, which is better for newbee's than many other sources.
BTW, 15 pounds of honey, in a 5 gallon must will get you to about 14% ABV. You can add a little more if you wish, but be careful there. It's better to have the yeast fully ferment and then add honey to get the sweetness you will want than to try to do it at the start. I'd also advise not back sweetening to what you want, but go a bit short of what you want. Mead will age out so that it appears to gain sweetness even though it technically doesn't. If you back sweeten it to where you want it before bottling, chances are it will be too sweet a year, or three, later.
Also, give the mead TIME. 9-12 months for a 14% mead is not uncommon. The longer you give it before going to bottles, the better, in fact.
Above all else DON'T heat the must up beyond 100-110F. Don't boil it, or pasteurize it (neither is needed, and you'll lose a lot of what makes the mead great if you do). I'd also go with a later honey harvest if you have a choice. Late season honey is typically stronger in flavor than mid season harvests.