The talent and experience he gained turned him into someone who could make the Destiny of Fate Supreme Metac from scratch. So even without making a single change in the River of Fate, he could tell which change was a good idea and which was suicidal.
He could also tell that reducing the chances of a heavy backlash was the highest priority, and he knew that to do this, he had to limit the target as much as possible. In general, the weaker the target, the better. And the less outrageous the change he wanted, the less the backlash for failing.
Apparently, his personal strength didn't affect the size of the backlash at all. The only thing his strength would affect was the chance of success for the change in destiny he initiated.
