I would bench test. Take say, 5 samples of a known volume (say, 50 CCs) and add known quantities of honey to each sample (say, zero g , 3 g, 6, g , 9 g, 12g stir to dissolve and taste. Let's say that 9 g is too sweet and 6 is not sufficiently sweet for your taste, then take another set of samples and add 6.5 g, 7 g, 7.5g 8 g and 8.5 g and stir to dissolve and taste. Let's say that 8.5 g of honey in 50 CCs is what you are looking for, then divide your total volume of mead by 50 (let's say that 5 gallons = 18925 ccs, so divided by 50 = 378. You then need to multiply your 8.5 by 378 to determine the total volume of honey you want to add - and that is 3.213 kg (or about 6.5 lbs of honey). In my imaginary example this will raise the gravity about 45 points. That would be tooth scrapingly sweet...You might find that you prefer 2 g of sugar or .5 g . The principle is the same - bench testing is the answer.