Calculating ABV after adding fruit puree to fermenting wort

I'm currently fermenting a gluten-free strawberry IPA.
The wort went into the fermenter with an OG of 1.050 (a little lower than I'd aimed for, but non-traditional grains can be less predictable).
Three days later, the yeast had been working, and the gravity was down to 1.021. I stirred in a few pounds of strawberries I'd pureed in the blender, and the gravity climbed to 1.033, presumably due to the addition of sugars from the fruit.
My question is about how to calculate the ABV when I check the final gravity before bottling tomorrow. My thought was to calculate the amount of alcohol created during the fermentation prior to adding the puree -- 1.050 to 1.021 --> ABV=3.81% -- and add it to the calculated alcohol from after the addition of strawberries to the final gravity -- 1.033 to (let's say) 1.015 --> 1.58%). If that approach is correct, my beer will have an ABV = 3.81 + 1.59 = 5.4%.
So, is that an accurate way to calculate my ABV? Or am I completely off track, having scuttled any hope of accurate calculation when I added the strawberries?

 

Thanks for everyone's input. It sounds like I can get a close, but probably not precise, ABV with what you've provided.

Grain bill = 7.7-lbs buckwheat (from 10-lbs sprouted, then oven baked brown and loose particles separated/removed), 1.5-lbs steel-cut oats, and 2-lbs of wild rice blend. I also boosted the fermentable sugars with 5-lbs of dextrose (ending up with 1.050 OG, as I noted above). This is similar to a GF Heady clone I recently attempted, but with some adjustments in the process due to lessons learned from that one (it's delicious, but I ended up with way too much gooey sediment and and lost volume because of it).
I've made a few GF beers now, and have found that the final product is usually good, if not excellent; but they require a lot more work, mainly because the homebrew suppliers don't sell most of the grains necessary, so I have to prepare them all myself.

 

That is just what I've been looking for! (many Google searches, but never found that site.)
Time is money, so I'd rather spend $50 than hours of my time.
I tasted the raw beer coming out of my fermenter today, and although I might add a little more lactose to boost sweetness next time (it's a little tart, probably from the fermented strawberries), it's promising to be a very refreshing (dare I say sessionable) beverage - a month of bottle conditioning should perfect it.. If you're a GF home brewer, I can share the recipe.