You can backsweeten any time after the wine is clear, and can do it in stages.
Math is a starting point. When anyone says, "backsweeten to taste", it's a literal statement. No one can reliably tell you how much sugar to add, as your liking is different from everyone else's.
Add sugar, stir well, and taste. Repeat until you are satisfied.
Wines meant to backsweetened are typically higher in acid, and require sugar to balance the acid. When tasting, consider how sharp the acid is on your tongue. More sugar will reduce that perception.
If you are unsure if the wine needs more? Let it rest a week or two, and taste again.
Don't worry about running the wine. Take your time, add sugar in small increments, and stir well.