It's roughly 1 pound of honey to 1% abv in 5 gallons, 15lb will get you around 14% and 18lb about 17%.
If looked after, WLP099 will consume all the sugars you have available at leave you with a very dry mead. 15% is no problem for that strain, and 18% is achievable with care.
The sweet mead yeast WLP720 will stop around 14-15%, although actual performance depends upon how much you baby sit it. So it could ferment out completely or leave your honey slightly sweet, although I doubt it would be anywhere near 1.023 unless you add the extra 3 lbs to take it up to 18lbs.
The good thing is that the WLP720 is self limiting. So in the event it does ferment out all your honey, you can keep adding more honey until the abv prevents the yeast fermenting it, so then the honey is then going to backsweeten the mead.
In your shoes, I'd start with 15-16lbs of honey, ferment with WLP720 and then after fermentation is complete, decide if I need to add more honey. Note that adding more honey may restart fermentation so be sure to leave for several weeks before bottling. (This is usually not an issue with meads, with tend to sit for months.)