From a strictly mathematical approach, and by using the approximation that 12 pounds of honey at the default 79.6% sugar concentration from the mead calculator is equal to 1 gallon of volume, I used the blending utility to calculate a default honey SG of 1.44. I don't know how that relates to water percentage, but if you found the weight in grams of any volume in mL of your honey, you could divide the weight by the volume which would give you the SG.
Once you have this number, you could put that in the required value of the blending utility with 1 as value #1 and 1.44 as value #2, and 100 gallons as the target volume, and the number that appears in volume #1 would be the percentage of extra water that would have to be added to the "default" honey to give you what you have.
Granted, this is based on a lot of assumptions, but it will at least give you some idea.