I usually use a RDA to test my new recipes but also a sub ohm tank since that is what I use 95% of the time. I usually mix 14 ml into a 15 ml bottle, the extra 1 ml of space is just to give it some room to give it a good shake.
The difference between mixing a 5ml test batch to a 14ml is not much, usually about .30 cents more at the most for a 8 flavor recipe and much less than that for a simple recipe. I like to have enough to try after a shake n vape in the RDA first and if I like it there then maybe about 3ml in a sub tank. If it is not too my liking then I just let it steep for a week or 2 and try it again. A lot of recipes have really changed from not to my liking to a possible all day vape after they have had time to fully steep. Some have actually changed from tasty to ehhhh, need to tweek the recipe after steeping as some flavors have become more pronounced to change the flavor profile too much.