This is just my stab at it. I posted it in the beginner forum thinking that this question had to have been a newbie question, no one replied.
Ok I searched for a bit for a ABV formula for times when something is added in the secondary and came up empty. Maybe I'm searching using the wrong words, anyway please help. Here is my best guess at it. Please help check my logic
OG = Original Gravity
PG = Primary Gravity (After initial fermentation before adding fruit)
SG = Secondary Gravity (After adding fruit)
FG = Final Gravity
PV = Primary Volume (Before adding fruit)
SV = Secondary Volume (After adding fruit)
(OG-PG) * 131 = 1ABV
This will figure out the ABV for the original brew. When we add the fruit it will dilute the ABV so the way I figure we will need to create a ratio to multiply the 1ABV by before adding it to the 2ABV.
PV/SV = 1ABVweight
Using this weight will determine dilution of the ABV
(SG - FG) * 131 = 2ABV
This will figure the alcohol increase between the addition of fruit until the fermentation is complete or stopped.
(1ABV * 1ABVweight)+2ABV
Unless I am missing something (definite possibility) this is the calculated final ABV