I created a lemon/ginger/carrot mead and started with SG 1.120. I used a mead calculator (https://gotmead.com/blog/the-mead-calculator/) to factor in the sugars in the carrot. However, the fermentation got stuck and the SG was 1.002 — completely dry. I expected the yeast to eat the shredded carrot, but alas, yeast are not piranhas; they have no teeth. I added one pound of honey (and 1/2 tbsp. Fermax and 1 tbsp. calcium carbonate) and stirred. It is bubbling once again but how will I calculate the ABV now? The formula (and calculators) only have slots for original gravity and final gravity. The yeast are Lalvin 71-B (14%).
If I understand correctly, given your recipe you cannot measure original gravity (because you added more sugars in the form of honey after fermentation began) and therefore you need to find a way to estimate original gravity. You will (eventually) measure final gravity and use a calculator to estimate ABV.
I would estimate the original gravity contribution of the late-addition honey by factoring in the points per gallon based on the amount of honey added and the number of gallons in your fermenter. While each batch of honey will be slightly different, a generally accepted value for one pound of honey is 35 points per gallon (you can search the internet for this easily enough).
In your case, I believe that you added one pound of honey to five gallons. This would mean that you would add seven points to your measured OG (1 pound times 35PPG divided by 5 gallons). So, your 1.120 OG would become 1.127.
If you want to be a bit more accurate, you would need to take into account that your measured original gravity got diluted a bit based on the fact that adding honey will have increased the volume in the fermenter.
Apparently it IS possible to get the ABV without knowing the OG by using both a hydrometer and a refractometer. See here: http://www.woodlandbrew.com/2013/02/abv-without-og.html
Late to the thread, but I did recently find The Mead Calculator thanks to a post on this other thread. The Mead Calculator allows you to plug in up to four different sugar additions, including for various fruits -- which has me stoked, as I'm having a lot of fun with melomels lately and was stumped for how to calculate final ABV.
Anyway, the calculator isn't perfect (different honeys can have somewhat different sugar content), but it should get you at least in the ballpark for your final ABV.