On the syringe there are 5 marks between numbers. Are those marks milliliters? I assume yes. And the numbers are centiliters.
Some old school syringes may have a secondary scale on them; marked as "CC"s. A "CC" stands for cubic centimeter.
1CC = 1mL (milliliter).
Small syringes (typically 1mL) can also have a second scale on them marked in "units" ("U"). There are 2 different types of units; "insulin," and "tuberculin." An insulin syringe is pretty straight forward, as there are always 100 insulin units in 1mL. An insulin syringe will be labeled as such on the barrel.
100U (units/insulin) = 1mL
Tuberculin units
are a different matter. A tuberculin unit is based on
the volume of the specific antigen it was calibrated to measure.
There is no fixed equivalent to mL; so disregard for DIY use. A tuberculin syringe will be labeled as such on the barrel.
What does all this mean? Simply; pay attention to
which scale (unit of measure) you are measuring with.
How to read/use a syringe is explained quite well (both written and a short video) here:
How to Read a Syringe
I have yet to find a dropper bottle dispense even close to 20 drops/mL. Usually(?) it is somewhere closer to 30 to as much as 45 drops per milliliter. Bottom line; don't use drops to DIY.
Do 20 drops = 1ml? and if so why did the second 30ml taste like the first.
Truthfully; DIY is
all about repeatability.
Because DIY has so many variables (personal preference, method of measurement, accuracy of data, precision of equipment, ambient temperature, ingredient temperature, calibration, tolerance of error, even atmospheric pressure... if you want to start splitting hairs), it really comes down to each DIYer's ability to duplicate his/her methods.
I have yet to find one single recipe that was "perfect the way it was posted."
Something always needs a tweak to be most pleasing to
my own taste buds. Once I figure out what that is; the skill becomes being able to repeat it. That means, doing the same thing... the same way...
every time. Find what works for you.... and stick with it.
Where having "conventions" (doing things the same way as others) becomes important; is when you want to share your recipes with others, or using other's recipes for yourself. I, personally, will never use "drops" as a method of measuring. But, I will also never disparage those that do. If it works for them... then all the more power to them!

It doesn't work for me; so I
choose not to
personally use it.
Remind yourself; that
everything (
including this post) is what works for
others. It is only through trial, and error (and success too), that you will find what works best for
you. 
As for aging (aka. "steeping;"
gawd I hate that word); it can never hurt. Aging usually won't hurt a given mix (though some concentrates can suffer "flavor fade"), and it has yet to save a truly "awful" mixture. But, it
can allow
some flavors to settle down (become less intense/dominating), and others to "bloom" (or "blossom")(develop and become more pronounced). I never write off as "bad;" a DIY mix before the two-month mark.