We recommend adding between 6% and 10% relative to the wax weight. So, if you have 1kg of wax, you should add between 60g and 100g of oil to it depending on your preference and test results.
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.