According to John Palmer's How to Brew, Lyle's Golden Syrup (that's a name brand but I assume it's the same or close to what you've got) is 82% sugar solids, whereas corn sugar is 92% ... but both are 100% fermentable.
In other words, the golden syrup has less sugar, gram for gram, so you'll need more.
So to calculate the amount of golden syrup you'd use the following equation:
<Weight of Corn Sugar you'd normally use> x .92 = <Weight of Golden Syrup> x .82
Or, to simplify:
Weight of Golden Syrup = <Weight of Corn Sugar> x .92 / .82
If you would have used 100g of corn sugar, you would use 112.2g of the golden syrup.