I would like to make a high ABV mead, and plan to divide the honey into 3 or 4 increments for the yeast. This has to be done so that the yeast are not under stress from osmotic pressure. However, I don't know how to calculate ABV if I am lowering and raising the gravity. How would one calculate the ABV?
Calculate the recipe as if you were adding all the honey up front. Also take an original gravity reading with just a partial amount of honey present; if you divide the gravity points by the number of pounds of mead you used, you should know how many more points will be added later on. For instance, if you were say adding 9 pounds of honey up front, then another 9 pounds later on, if the OG from the first 9 pounds turns out to be 1.063, then you know that with the entire 18 pounds it would have been 63 times 2 or 1.126. Or since 63 divided by 9 is 7, there are (perhaps) 7 points added per pound of honey, so if you added 7 pounds instead of 9 then you would be adding 7 times 7 = 49, or 1.063 + 1.049 = 1.112.
In a 5-gallon batch I think the typical number for most honey is about 7-8 points per pound. Multiplying that by 5, that translates to about 35-40 points per pound per gallon. Each particular honey is different, hence the range of 35-40 ppg.
With a bit of mathematics you can figure this out. And software helps, such as BeerSmith or BrewersFriend.com or similar.