I'm doing a recipe, and I'll use inverted sugar that I cooked yesterday. But I don't have idea how to handle the specific gravity. I tried to use the densimeter, but it's useless.
You could try this, if you have an accurate scale:
Dilute the syrup to a 10% solution, by weight. For example, 90 grams of water and 10 grams of invert sugar. Mix it very well, and measure the gravity with your hydrometer. The Brix/Plato scale on the hydrometer is the most useful here, as it shows percent sugar, as sucrose. You can use this reading to determine how much sucrose-equivalent is in your sample, and extrapolate to the undiluted syrup.
For example, suppose your dilute sample reads 8 Brix on the hydrometer. That means your 100 gram sample has 8 grams of sugar. Multiply by 10 to determine the percent sugar in the undiluted syrup: 80 grams. Hence your syrup has 80% as much sugar as the same weight of sucrose.