For bigger bubbles, use up to 2 teaspoons baking soda, keeping in mind there might be an aftertaste. Do not use baking powder!
Substitute golden syrup for the corn syrup if desired.
You can also check with a probe thermometer. Some folks say you can simply go by appearance, and when the syrup goes from clear to slightly golden, it's done, but that requires a certain amount of experience. Another method to gauge the temperature is by dropping a little bit of the molten syrup in water to see if it instantly turns into rock candy. That will work, but since thermometers aren't expensive, and every kitchen should have one, that really is the way to go.