Someone correct me if I'm wrong but I think Redstone uses Montrachet yeast which typically goes to about 13% ABV.
If you want to know for sure how much honey to add, take a bottle of the Restone mead and measure the gravity. That is where you want to end. For 1.015 you would then add honey to your mix to get the gravity up to about 1.115, and it should finish close to 1.015. That would typically be close to about 3.25 pounds of honey per gallon of mead. Since honey can vary quite a bit, it is probably a good idea to use the hydrometer to guide you or else you could be quite a bit higher or lower than 1.115.
Of course, there is another way to do it to get the exact level of sweetness you want. You ferment a batch completely dry, then stabilizing by adding potassium metabisulfite and potassium sorbate. This will then allow you to add honey without it being fermented by the yeast and you can add it a little at the time to get the sweetness exactly where you prefer it. I happen to favor this approach as it gives you more control over the final result.
In this case, I would probably add a bit less than three pounds per gallon shooting for a gravity of 1.095-1.100. You can use the Montrachet yeast although if can produce some off flavors if it get too warm or is undernourished. There are many other yeast that work well, and which may be less finicky, but I believe that Redstone uses Montrachet and if you want to get that exact flavor, it may be the one to use.
You may want to spend some time looking at the mead
NewBee Guide. It will give you a lot of information to help you get what you are looking for.
I hope that helps.
Medsen