I believe PPM is grams per 1000 Liters.
So,
100 grams of nitrogen in 1000 liters would give 100 ppm
50 grams of nitrogen in 1000 liters would give 50ppm
however, to get actual grams of nitrogen, it depends on your fertilizers. For example, Calicum Nitrate, which is 15.5-0-0 NPK is 15.5 % nitrogen. so to get 100 grams of nitrogen from this, you would need 100g / 15.5% = 645g of Calcium Nitrate).
So, 645 g of Calcium Nitrate with purity of 15.5-0-0 will give 100g of pure nitrogen.
Dissolving 645 g of Calcium NItrate into 1000L of water will give you 100 ppm.
Fairly sure this is how it works :)
It is a bit more complicated with P and K values, as they have normalization values
P is only 43.6% pure
K is only 83% pure, so this should be taken into considering at the beginning before caluculations.
Also, from the allhydronpics blog above, he just released a Nutrient Calculator!!
http://allhydroponics.blogspot.com/2010/07/preparing-your-own-hydroponic-nutrients.html
check it out, this makes things a lot simpler, all you need to do is plug in target PPM values, and the calculator spits out the results. Keep in mind, the Calcium Nitrate on the top of his list is actually 11.8-0-0, where as the more popular brand (the one I have) is 15.5-0-0 and is on the bottom of the list (have to scroll down) labeled as Yara brand
http://allhydroponics.blogspot.com/2010/07/preparing-your-own-hydroponic-nutrients.html
check it out!