I derived this earlier, but wanted to post it before Lumos. I am posting herre because Bottom!Snape is a major factor. I have yet to actually test it...
Where...
B = Number of times Severus bottoms in the fic C = C is 1 if Snape is possesive and controlling in the fic; otherwise C is 0 D = Number of Death Eaters Severus has slept with E = Number of sex partners Harry has had before Severus F = F is zero if Snape is Harry's father; otherwise F is one G = Number of times Snape fucks Harry with Harry on top H = Harry's age in the fic when Harry and Severus actually have sex; if the fic happens over time use the average age I = Number of inches Severus is taller than Harry L = Number of words before the author reveals Snape's loyalty M = M is 3 if a good portion of the fic centers around mind games, M is 2 if there is some mild mind gaming, otherwise M is 1 P = Number of special potions Snape brews for Harry R = Number of Snape broomrides S = Number of times someone speculates about Snape's sexuality T = Number of third parties added to the Snarry mix V = V is 3 is Snape has willingly had sex with Voldemort, V is 2 if Snape has unwillingly had sex with Voldemort, V is 1 if Snape has never had sex with Voldemort X = Number of scenes which involve any of the following: BDSM, masturbation, toys, or other stuff I like Y = Number of scenes which involve any of the following: animals, urine, shit, or other squicky stuff; Y = 1 if none are present
Notice that Snape having certain qualities can counterbalance a young Harry; however, the equation is optimized when Harry is about 25. The main part of the equation also becomes zero if Snape is Harry's father. Technically a loyalty amibugous Severitus fic that involves Possesive!Snape who is really tall, rides lots of brooms, and fucks Harry in certain positions could get a positive score, but that's really not my thing.
Note: This only applies to longer fics, not drabbles; drabbles have a completely different standard