Specific gravity is a ratio. For an actual definition and explanation you can read this:
Specific Gravity. In the more real-world terms of the average DIYer it is comparing the weight of water (given a value of 1milligram/milliliter) to the same volume of the ingredient in question. If it weighs less you will see values of less than 1 (e.g. 0.923), or if it weighs more than water it will have value greater than 1 (e.g. 1.264).
@Capt.shay .shay has compiled a pretty commonly accepted list of accepted "average" weights for common ingredients like PG, VG, and nic at various strengths and bases
here in his measuring by weight tutorial.
DISCLAIMER: everything following is MY opinion, and mine alone. I encourage all who read it to question it all, do their own research, and come to their own conclusions. The following are only my own conclusions to this point and may well change.
In my mind (and I am only speaking for me) the biggest/first one to throw calculations off is VG; simply because it most often has the greatest difference compared to water (water being equal to 1 while VG is equal to 1.26) and it often is one of, if not
the, single biggest percentage of a recipe. If you have your VG value set to 1 gram/milliliter instead of 1.26 grams/milliliter you are going to be 19% short of your total VG volume. If you are mixing high VG recipes it can throw your end result noticeably out of wack.
Flavors are a tuffy. Every flavor is going to be a little bit different depending on what is actually used to make the flavor concentrate. Flavors with a lot of alcohol will weigh less than 1. Others can weigh more. The base/carrier of the concentrate can also have a substantial affect. Those made with PG as the carrier (most common) will be lighter than those made with VG (less common but becoming more available and popular).
Most DIYers who measure by weight will input a value of 1.038 grams/milliliter (give or take a couple of hundredths)for PG, and a value of 1.26 grams/milliliter (again give or take a couple of hundredths) for VG, and a third value specific to the nicotine concentrate they are using (see the list generously provided by Capt.shay).
Then for flavor; DIYer's will do 1 of three things (listed from most common action taken to least common <note that I said "common;" not "correct" or "accurate">):
1) Give all flavors a common value of 1g/mL. This is easiest and the highs can "kinda/sorta" balance out the lows. As this is the most common practice; it makes sharing recipes the most 1:1 repeatable.
2) Give all PG based concentrates the same value as PG (1.038g/mL) and all VG based concentrates the same value as VG (1.26)... with the feeling that it gets them a little closer to the truth. Whether they are right or wrong really boils down to the individual ingredient. Not as common a practice as #1 but still common. Also will start to introduce
minor variations when sharing recipes with those using other methods.
3) Research each ingredient for a MSD record (or similar) that will state a specific gravity for the ingredient in question and then use that value. This is probably a little closer to "accurate/true" SG than 1 or 2 but harder to find. Some vendors are quite open to sharing this info while others publish nothing; leading to choosing one of the other options for
select ingredients. So it is going to be real hit and miss and will introduce further discrepancies when sharing recipes. Much less popular than #1 or #2 as it requires a LOT more work with not a lot of extra benefit.
4) I guess, for thoroughness, I should mention a fourth option (though I haven't heard anyone say they actually do this) would be to physically measure out and weigh each and every flavor concentrate in one's personal inventory. Has potential to be most accurate but you are, at the same time, potentially introducing the greatest chance for error; as it is completely at the mercy of the quality, and condition, of the measurer's equipment and the care with which they carry out the measurements.
AND... none of this takes in to account the correctness/accuracy of one's actual scale (how/if it is calibrated, it's designed resolution meaning acceptable variance which is often displayed as a +/- value) OR environmental conditions (primarily temperature, and secondarily atmospheric pressure/height above sea level), OR the acceptable variation of each batch of said ingredient from the manufacturer. Even standard ingredients like PG and VG aren't really standard. One vendor's VG will weigh slightly different than another vendor's VG.
You can see how quickly a person can get lost in the petty, minuscule, details of being "accurate." This is why the majority will settle on an "average" value and move on.
Please understand; I am not suggesting one way is "more right" than another. Neither am I sitting in judgement of anyone doing ANYTHING. What ever works best for
you IS YOUR right way.
Remember that DIY is closer to cooking than it is to pharmacology. When was the last time you, or your Mother, or ANY professional chef you know/have seen, has checked the accuracy of their measuring cups or spoons? The one exception I can think of
m i g h t be a professional baker who is dependent on his/her scale.
Bottom line is: "Close enough for
you is close enough."
Cheers!