Plain or "Fragrance" oils do not disclose the ingredients used in making. You may not be getting the concentration you need, nor a purely "Lavender" ingredient makeup.
Essential oils are highly concentrated. They are what we would call "Pure" - usually completely pure of the ingredient mentioned on the label.
If a recipe calls for peppermint oil, you can use peppermint essential oil instead (usually people ask the vice-versa), however, because the essential oil is less diluted than the plain (or "fragrant") oil, it is not possible to know exactly how much less of the essential oil will work for the recipe.
The highest concentration of essential oils in your blend should be less than 20% proportional to the base alcohol or oil, which is actually the classic French perfume concentration ratio.
(20 drops total (adding up all essential oils used) for 4 mL base)
I'd say since it is a cosmetic recipe (perfume?), mix in adjusted to preference, but it will more likely be significantly lower amount needed.