We recommend adding between 5% and 10% relative to the wax weight. So, if you have 1kg of wax, you should add between 50g and 100g of oil to it depending on your preference and test results.
As a general rule, the more paraffin wax that is in your blend, the less perfume you need to add. For example, it is not unusual for a perfectly good paraffin wax candle to contain 5% or less fragrance. Indeed, adding more than 5-8% perfume to a paraffin wax candle can lead to significant burn issues and is a common error made by beginners. Vegetable waxes do not produce the same level of hot scent throw and form melt pools more easily, so are usually scented at 7-10%.
If you do not get strong scent throw at 10%, with a correctly wicked candle, you are unlikely to improve this by adding more oil. You will need to choose a different oil, or try a different wax. Not all perfumes perform equally well in all waxes. Testing is recommended.