HSI is calculated based on 6 months storage at 68F. For instance, a hop that starts at 10% alpha with a HSI of .250 will have 7.5% after 6 months at 68F. However, storing those same hops at 28F for 6 months will leave you with 9% AA.
The other thing to keep in mind is that it varies by variety and harvest conditions. So it’s only applicable if you have the hops analyzed like you did.
If you decide to pony up the dough for another round of analysis next year, you should spring for the basic oil content test as well. The oil content rating is more useful than the alpha/beta ratings when it comes late hopping. I ponied up the dough for the “Hop Profile Package,” which includes the brewing values (alpha, beta, and HSI), oil content percentage (a.k.a. milliliters of oil per 100 grams of hops), and the volatile oil analysis (gas chromatograph analysis of the volatile oils). The only other value that I would have liked to have obtained was co-humulone as a percentage of the alpha acids.