Here's where you went off the rails:
And then calculated g/L in the 37% solution:
1280 x 37 / 100 = 473.6 g/L
I have no idea where you pulled the 1280 figure from. [Edit: actually, I do know. You used 1.28 g/mL as the density for sulfuric acid instead of the correct 1.84 g/mL]
The density of pure sulfuric acid is 1.84 grams/mL
Grams per liter in pure sulfuric acid is 1840. Grams per liter in 37% acid is 1840 grams * 0.37 = 680.8 (not 473.6 grams).
Also, don't make 100 mL of 0.12N acid solution, it makes the volume of 37% acid to add too low for a really accurate measure unless you have some more advanced equipment. Make 1 liter of 0.12N acid solution instead.
The molecular weight of sulfuric acid isn't 98 grams per mole, it's 98.08. It's only about a 0.1% error, but let's use the right value.
With sulfuric acid, 0.12N is 0.06M due to the acid being diprotic. So to make a liter of 0.12N sulfuric acid, you need 98.08g * 0.06 = 5.88 grams of pure sulfuric acid.
Now we're down to the crunch. Since the 37% acid contains 680.8 g/L of sulfuric acid, how many mL of 37% acid do we need to get 5.88 grams of sulfuric acid? That's just ((5.88 / 680.8) * 1000) = 8.636 mL of 37% sulfuric acid.
So, you would add 8.64 mL of 37% sulfuric acid to 250 mL of distilled or deionized water and mix. Then top off the solution to 1 liter with water, mix and serve.
Analyzing your results with the incorrectly made acid, you need to multiply all your nicotine concentrations by 1.4375 to get the actual concentration. So your 72 mg that tested at 64 mg would be 92 mg. Your 24 mg that tested at 20.2 mg would be 29 mg. Clearly something isn't right. That something is most likely the assumption that the 37% sulfuric acid you bought is really 37% sulfuric acid. It's looking more like it's considerably lower, maybe 26% or so. I've seen battery acid formulations that run as low as 25% and as high as 37%. Really depends on the manufacturer.
It's not a rule, but it's generally held true in my experience that muriatic acid (hydrochloric acid) available in hardware stores for driveway cleaning and such is much more accurately made. I've got a gallon of 31.45% muriatic acid. Since concentrated 37% hydrochloric acid is 12.1M, 31.45M hydrochloric acid would be 10.285M. I tested this stuff and it came out to 10.2M, barely a 1% error.
Somebody check my math, its late and I flew from the West Coast to the East Coast earlier today so I'm extra tired.
Thank you very much for the elaborate response, DVap.
The 1280g/L is from the manufacturers product homepage. I see now, that I propably misunderstood the formulas input definition. Isn't the formula "density ( g/L) x % by mass / 100 = g/L of H2SO4" meant to find the actual weight of H2SO4 molecules in solution?
I'm making smaller batches to sanity check the mixing. When I can mix 100mL with good results over a few trys, I'll mix one liter, to raise accuracy.
Funny thing, I actually reached that number, 8.64, myself, when trying to calculate by camparing and scaling N value for 95.8% solution, where I knew the N value. But that mix testet the nic base way to high, as you also predict it would.
So, the truth is some where in between (isn't it allways ).
I have written to the manufacturer to get some answers on the tolerance of the battery acid.
And maybe I should just buy som hydrochloric acid instead. That's also easier to find.