It’s important to add the vinegar mixture to the rice as soon as it has finished cooking. If the rice sits too long it is very difficult to evenly season the rice. Depending on the altitude and climate, you may want to alter the amount of water used when cooking the rice. If you're living somewhere humid, you might want to drop the water a little bit. If you live somewhere really dry, increase it just a little bit. Ultimately, with sushi rice we want to notice each individual grain of rice while we're mixing and eating it.