If you're making a ~5 gallon batch, I would use 12-14# of honey initially... It's better to have it go dry, while the yeast is still alive, adding some of the reserved honey until it cries 'uncle' than have it finish too sweet. You can simply let it ferment fully (it could go down to .990) and then add ~1/2# at a time at that point... If you want to short the batch for water, so that it's a bit stronger, that could be ok, just be careful. Keep in mind, 12# of honey is 1 gallon of volume. Which means, 1# takes up about 10.6 fluid ounces for volume... So, if you reserve 1-3# of honey, you're only talking about ~1 quart of volume less. I would split the difference and only short the batch about 1 pint, if at all.
I tried to get my first traditional batches to finish to a target ABV/FG... I added too much honey, so it's not going there. I wanted in the area of 1.010, but I'll be lucky if it gets to 1.020...
I wouldn't use potassium carbonate unless the must has a really low PH... Like under 3.2... Use some test strips to determine that before you add any pot. carb. to the batch... I've not needed it in any of my batches of mead. The only thing I needed to add it to was my hard lemonade, due to how much acid is in lemonade...
If you want, you can make a bit more than 5 gallons of must, initially, and reserve 1-2 quarts... IF you do that, just be aware that you'll need more honey in order to get where you want it... I would simply not worry about it that much...
IF you have, or get, a refractometer and use one of the tools to offset for alcohol, you'll lose much less during the process. Or just sanitize everything that will touch it once you start the batch and simply pour the hydrometer sample back in. You'll not lose all that much from trub when racking. You'll lose more from blow-off (if you use a primary that's a bit tight, like I did) and hydrometer samples. I think it would be safe to feed, degass, and aerate each day for ~4-5 days, then take another hydrometer reading. If it's getting close to the 1/3 break, then take readings each day after that... Since you're OG will be in the area of 1.100, figure the 1/3 break as being around 1.063... at that point, you can leave it alone to finish... It could take the balance of a month before it's actually done fermenting. Take a reading at that time, to confirm. I would add the additional honey, a bit at a time, while still in that carboy/fermenter... Once it's finished, and stable, rack.
I would also stop adding honey when you hit about 1.006... That way, it will get better with age and you won't have issues.
This is how I plan on making my future batches... Plan on letting it ferment to dry, with yeast still viable, then add more honey to get it to the tolerance of the yeast, and beyond, then add a bit of sweetness before aging...
You could also tell the people you're giving bottles to (give them two if you can) to give it a few months, then open one. Let the other go until the following fall before having it.
