To round a double in Dart to a given degree of precision AFTER the decimal point, you can use the built-in solution in dart toStringAsFixed() method, but you have to convert it back to double
void main() {
var a = 11.23984948449;
print(a.toStringAsPrecision(3)); // result are : 11.2
print((4321.12345678).toStringAsFixed(3)); // result are : 4321.123
print(a.toStringAsFixed(3)); // result are : 11.240
print(a.toStringAsExponential(3)); // result are : 1.124e+1
}