Capsaicinoids are present in the flesh/skin/placenta of any given chile. If the skin of the pod is measured for capsaicinoids there would be basically none. The placenta (the ribs/membrane that the seeds are attached to) contain the most ppm of capsaicinoids.
For simplicity, the ppm of any given chile variety is averaged and the ppm are converted to the Scoville heat scale. The scale is a generalization that habaneros "generally" have (whatever the number is)PPM of capsaicinoids that equals 250,000 to 400,000. Red Savinas have more ppm capsaicinoids so they get rated up to 500,000 SHU.
There's more information about capsaicinoids ( capsaicin is one of many capsaicinoids)
Here-
And check out ScottRobertsWeb for a comprehensive list of all kinds of chiles, sauces, extracts, and other capsaicin-laced products. There's a link in the above link...
I can't remember the equation, but the American Spice Trade Association adopted SHU as a measure of pungency about 30 years ago when HPLC testing came into use. HPLC measures the PPM of capsaicinoids in whatever (chile, sauce, extract, Pure Evil) and that ppm number converts to the SHU rating.
Side note- the ASTA uses pungency testing on cinnamon, paprika and several other spices and products. SW Bio-Labs in New Mexico is within a stone's throw of probably 100+ chile growers as well as the NM Chile Pepper Institute. They do hundreds of pungency tests a year. Chile processors have to have an accurate rating of how hot "this batch" of ground chiles is. Chile heat can vary probably up to 50% from year to year and field to field. Imagine what would happen for large processors (I'm talking Major Food Manufacturers that are using chiles much more nowadays) if the pungency of one batch of is way hotter than the last. They have to have consistency of heat and knowing what the pungency/SHU of their ingredients is critical to an consistent end product.
