*EDIT - I realize now that I may have misunderstood, if you are saying let the beer reach final gravity, then add fruit and take new gravity of total volume, then you should be fine just adding that gravity to OG. You would just have to make sure the fruit is mixed well into solution or it will give a wrong gravity (so doing it the below way may be more accurate) but I’ll leave the information below as it may be helpful for others*

Unfortunately you can’t just calculate the gravity of the fruit and add it to the OG, that’s going to give a higher abv than is correct (unless your fruit adds no additional liquid to the fermenter) In fact, if the sugars in the volume of fruit liquid are lower than your OG, your final ABV will actually drop as a result of adding fruit. You need to calculate it as a percentage of total volume; something like this:

New OG = (OG*A)+(FruitOG*B)

Where

OG =original gravity

A= % volume of original wort

FruitOG= gravity of fruit addition

B = % volume of liquid added by fruit

This won’t be 100% accurate, but is close enough for a homebrew level. I calculate the gravity of the fruit by taking a sample before I add it to the fermenter, but there are also calculators online I believe to give an estamate.

For example (because I know I’m bad at explaining)

If your OG is 1.055, 5 gallons

And you add 1/2 gallon of fruit juice at 1.045:

Take .5/5.5 = 0.09

5/5.5 = 0.91

So, (.055*.91)+(.045*.09) = (.05)+(.004) = new OG of 1.054

Use this number in conjunction with you FG post fruit addition to calculate ABV

At least this is how I’ve been doing it, someone please step in and correct if I’ve been doing it wrong