First, you must adjust the boiling point of water. If water boils at sea level at 212 F, what you must do is boil water and figure out what the temp is that the water boils at in your house. Then make the adjustment. For example, I live at a high altitude, water boils at approx 200 F. Therefore I must adjust any recipe I use by 12 degrees. So when I make turtles, the caramel must boil to 242 or so. I boil it to 230 F.
Secondly, when the temp reaches 230 F (I am using caramel as an example but this applies to any cooked sugar) then I remove it from the heat. Not a moment longer!!
For fondant I don’t paddle by hand as it is too hard!! I have a bosch kitchen machine and I use my dough hooks to paddle it. So if you have any kind of kitchen mix master that is strong enough to knead bread, use the same dough hooks to mix your fondant. I just watch very carefully until the fondant becomes white and is the consistency that appeals to me.