Alcohol does strange things.
One of those means that the ABV calculated by good old simple OG-FG * x is pretty good up to about 6% ABV when the results start to drift. This is not new and was observed by Daniels in his classic Brewing Great Beers. Having brewed a RIS with some pretty stupid gravities I found calculating using this method was the better path.
So here it is (the first of a number of online calculators)