I've only formulated a couple of recipes, and modified others. None were fruit beers. When I do create a recipe, I use the calculator on BrewersFriend to check the expected results. But I prefer to use established recipes. I think it's a good idea to start with proven formulas.
Thanks for the BrewersFriend calculator. Not sure on the amount of fermentables in the fruit, but looks like I'm safe doing 2.5 gallon batches and hitting my target abv. I'm going to go for it, and make some guesstimates on what to do. Shouldn't be too hard to make something that tastes decent.