Mead and specific gravity / abv?

probably been asked before but…

lets say i made a must with 4 gallons water + 10 lbs honey + 5 lbs blueberries.

i pitched 2 sachets of lalvin 76b-1122 yeast, alcohol tolerence is about 14%.
fermentation temp about 68f.

in one month i need to add 5 more pounds of honey and 5 more pounds of blueberries. i will be removing the old blueberries first.

i will probably top it off to 5 gallons at this point too.

after a month i rack to secondary and then continue fermenting for about 4-6 months before bottling.

question: how do i calculate the gravity at the addition of the new honey, fruit and water?

question: i am concerned about alcohol tolerence for this yeast and maybe the new honey causing osmosis (?) issues for the yeast. would the additions put the potential alcohol over 14% or strain the yeast?

question: if so, would pitching a pack of yeast with say 18% alcohol tolerence with some nutrient work to finish? i would like a sweet to semi dry final product. sweet is preferred.

thank you in advance for any help…it truly is appreciated.

The berry’s will be the difficult part. Sugar content will very with the climate and verity. If using frozen you may be able to calculate it with the nutritional label.

The honey you can calculate with any brewing software. Or longhand.

http://home.comcast.net/~mzapx1/FAQ/MeadOG.pdf

If you have a refractometer, you can take a reading with that and a hydrometer and caluclate the ABV.

http://www.brewheads.com/refract-getabw.php