Calculating the specific gravity of your nicotine solution.

 
Nicotine Propylene Glycol Vegetable Glycerin Specific Gravity DIY - Liquid Barn
 
If you have a mix of Nicotine that is not listed above the calculation is fairly simple:
 
48mg VG based nicotine solution. That's 4.8% pure nicotine and 95.2% pure VG. Pure nicotine weighs 1.01g/ml and Pure VG weights 1.26g/ml. Let's assume we have 100ml for simplicity with decimal points.
 
4.8 x 1.01 = 4.848 grams.
95.2 x 1.26 = 119.952 grams.
If we add these two together: 4.848 + 119.952 = 124.8 grams.
Divide it by 100 and we have our weight per ml for 48mg nic.
124.8 / 100 = 1.248 g/ml.
This same breakdown can be done for 50/50 blends, just know that the non pure nicotine portion of your base is split 50/50, for example, 60ml in 50/50 mix = 6% pure nic, 94% left over = 47% VG, 47% PG.
 
When it comes to flavors, there are a couple of thoughts/theories surrounding mixing by weight when dealing with flavors. Many people enter the specific gravity as indicated on an MSDS for every flavor that they have. I feel this is a complete waste of time. After searching hundreds of MSDS sheets for flavors, all flavors fall into a range of .93g per ml to 1.07g per ml.
 
In order to achieve personal consistency for your juices though, all that is relevant is that you use the SAME value, every time for any given flavor. I simply use a value of 1. This means that whether or not my flavors actually weigh 1 gram per ml, I always pretend they do. Percentages in any given recipe of mine will always be the exact same for me, however, if I am trying to recreate someone elses recipe, or they are mixing mine, there's a chance I'll be off a little bit (no more than 7% which is about the margin of error most people get when mixing by volume).