I would say YES. The more base, the more flavor you must use to flavor that base.
If I was making a TEASPOON of chocolate milk, I would add one drop chocolate to teaspoon milk. If I was to make a glass of Chocolate Milk, One drop wouldn't do it for me.
AH, but, as the OP said, he used 15
percent...that means of the total volume, the amount of flavor used was 15
percent.
- 15% of 1ml = .15ml
- 15% of 20ml = 3ml
The ratio remains the same.
To the OP, how are you measuring? Drops or actual ml? As drop size can vary depending on not only the bottle/dropper but also the pressure used when squeezing, you may not always be dispensing the same amount.
The MOST accurate way of measuring is, of course, by ml, but it can get hard to get .8ml, for example, unless your tool (syringe in my case) has the needed graduations (the smallest on my syringe is 2/10ths).
S