This is a staple in our house.
When preparing a batch of sushi rice, use 80 ml of rice vinegar, 2-3 tablespoons of sugar and 1 teaspoon of salt for 3 rice cooker cups of rice (480 ml), and it will have just the right amount of sweetness.

If you dissolve the sugar while heating the vinegar, the vinegar flavor will dissipate and become weak, so let the sugar dissolve naturally.
The vinegar sauce is not that sweet. Adjust the amount of sugar to taste.
To reduce the salt, use 1 teaspoon for 80 ml of vinegar. Recipe by Yuuyuu0221