I would liken it to trying to measure a baby's BMI. It's probably not going to be accurate, although you can often have a good guess, you can look to the parents for some clue although that too can throw you off and it's going to change so quickly that it's best to wait.
My hair is in the 4s, low in porosity, high in density and coarse.

Love: QB - all of it, Bobeam - shampoo bars, Darcy's Botanicals - Pumpkin Seed Conditioner, Coconut Cupuacu Pomade, KBN - Shealoe Leave-in, Oyin - Juices & Berries, Kinky Curly - Knot Today, Ayurvedic treatments, my Denman and the cloud of kinky goodness on my head that I get to play with!