I am of the "Start low and work your way up" school. Flavors, when used to excess, can actually become muted. If you don't know where this threshold is (for you) you can overshoot and add all the flavor you want and it will not be pleasing. Diluting also can be more difficult to accurately calculate for replication.
Aging ("steeping") requirements are affected by ingredients (tobaccos, custards, chocolates, etc. require longer times), volumes (the larger the batch-size; the more aging
can required), and personal preference.
Whenever I am working with new/unfamiliar ingredients; I always start with small "test" batches and taste frequently (immediately after mixing and then every couple of days) until I learn whether it is a shake-and-vape, few-day/short-term rest, or multi-week rest ingredient/recipe.
As
@Train2 suggests; the use of a recipe calculator is invaluable to know what that "6.5%" really means based on the volume you are trying to mix. As for syringes; a typical medical-grade 1milliliter syringe (like a diabetic syringe) will be graduated (the markings) in 0.01 (1/100th) mL increments. That is plenty precise enough for most recipes. Larger syringes will not be as precise. When using syringes I always tried not to use a larger syringe than necessary.
The other common method of measuring is tomeasure by weight and use a scale that has sufficient capacity (500 gram tends to be the most popular), and is scaled to read to 0.01 (1/100th) grams. This will give you adequate precision.
@Capt.shay has a great introductory thread on mixing by weight here:
Mixing By Weight: Basics 101
Both are perfectly fine for DIY and both have their fans. Several moths ago I switched from syringes to a scale because I had a hard time seeing those tiny little marks on syringes. I found the digital read-out much easier to see. But again; either is just fine.