It's actually not all that difficult.
The labels on the bottle are a percentage. PPM is also a percentage, but on a more accurate scale (parts per million rather than parts per 100).
So, if you do a little math you can figure it all out.
Basically you take the nutrient value on the bottle and multiply it by 10,000 to get parts per million. Then you divide that by the value of the total ml of solution divided by the ml of nutrient used.
So say you had a 1-2-3 nutrient. You add 10ml of this to 3770ml for a total of 3780ml (one gallon) of solution.
N=(1x10,000)/(3780/10)=26.5ppm
P2O4=(2x10,000)/(3780/10)=53ppm
K2O=(3x10,000)/(3780/10)=79ppm
Now, you have to understand how approximate these levels are. What is listed on the bottle is only what they "guarantee" to be in there. There could be more, and the nute companies are not obligated to tell us all their "secrets."
Another thing of note is that P2O4 is only 44% Phosphorus, so to get the actual level of Phosphorus ppms you need to multiply by 0.44 to get the right level.
Potassium is also listed as K2O which is 83% Potassium, so multiply by 0.83 to get the true level of Potassium.
Also, these values are listed in PPM but based on a conversion from an EC meter, so also... not exceptionally accurate. When the ppm is measured with a meter there's also unlisted Sulfur and other micro-elements that will cause the actual ppm readings to be about 15% higher than calculated. EC is going to be a more accurate way of comparing one system to another, but for the sake of estimations and simplicity you can ballpark your levels.
I do my calculations a little easier rather than figuring out the total volume of solution and all that... I mean, a spread sheet set up right could make this easy too...
What I do is take the nutrient value, multiply by the volume used per gallon in ml, then multiply by 2.7. This gives me a slightly higher ppm level than I would get with the earlier formula but that's cool with me based on the inaccuracy of the labeling. In reality my levels and t0rn's levels could deviate by as much as 10% from one another... so it's going to take a little testing and fine tuning to get things dialed in right.
Comparing my final EC with his final EC would be a good comparison. I don't think t0rn has that equipment yet, but I could be (I hope) wrong.