I guess I don't really understand your question. The sugars in honey are - for all intents and purposes 100 percent fermentable. Any yeast that you pitch in a must where honey is the source of sugar is 100 percent attenuated. But the only problem I see is that to produce a mead with sufficient flavor you will need to use at least 2.5- 3 lbs of honey per gallon. That means the SG of the must will be about 1.100 - 1.120 and that means that the potential ABV of that mead will be about 13 - 16 %. Will your safale tolerate 15 or 16 % ABV? If it will then no problem. If your ale yeast is not up to tolerating that kind of concentration of sugar or that amount of alcohol, then you might want to think about using yeasts designed to ferment wines (71B, QA 23, D47 , even champagne yeasts if you don't mind their aggressive nature and their tendency to blow off flavor and aromatic molecules. Dry wine yeasts are packaged to ferment 1-6 gallons of must without blinking..