It depends on what is in your must. If you are using liquids they will blend together very quickly and you can take an accurate reading. Immediately, once your must is well mixed.
Yesterday, I mixed up a kiwi rhubarb, with bananas. I let it warm on the counter with pectic enzyme. Once it was at room temperature I added the solids to nut milk bags to make it easier to transfer to secondary. I mixed the must to 1.100 and transferred solids to the bags. I moved the bags to another primary bucket and let them drain. The liquid from the fruit was 1.080. I added an additional 1 1/2 cups of sugar to the must to bring it to 1.120. Higher than I want, but with the hopes that it will balance around 1.100.
I admit I don't know 100% what my ABV will be. What I have found is that yeast do a great job of breaking down fruit and extracting what they can. A full bag of fruit will compress down to a handful of pulp after the yeast have had a few days to digest it. I could have pressed out more juice and gotten a perfect reading, but I really don't care to be THAT precise with a gallon of completely experimental flavors.
With a 6 gallon kit I am diligent on accurate readings and recordings, but they are VERY easy without solids. This one is a total experiment, so being less specific is ok with me.
As for Sour Grapes misunderstanding (as well as mine) we use three digits to minimize confusion, but it is also considered an increase from 1.080 to 1.110, and as the fermentation progresses the SG will drop below 1.000 at completion. As low as .990 for a very dry wine.
In your case there were more sugars added to the must with the raisins, and time could have allowed that to increase your SG. The thing to consider is your initial specific gravity is a measure of potential ABV. There are other factors involved that make it a rough estimation. The reason I want to know my starting gravity is to have a rough idea of ABV, but also to track the fermentation as it progresses. You will never know 100% only using a hydrometer, and for me 11-13% is what I aim for.
