Henk Kraaijenhof mentions the 10% rule in an article on hill sprints from 1986. At the time Henk was coaching Nelli Cooman who set a world record 7.00s in the 60m indoors eclipsing the East Germans. She famously trained by using the ramps in a local parking garage. He writes that a steeper hill causes poor mechanics, like low hips and insufficient extension. He refers to a study by Schmidtbleicher, Antoni & Dietz “Innervationsmuster der Beinstreckmuskulatur bei Bergauflaüfen” in Leistungssport 5/1981, pages 350-356. I have it but it is buried somewhere in one the many boxes still unopened from my last move. Henk recommends a maximum of 10 degrees, but for very short distances (10-30m) it may be steeper.
I have never really used the 10% rule in practice, which would require not only timing the hill run but also measuring the length of the hill and timing a flat run on a similar surface for the same distance. I just take the hill that is at my disposal and make sure that there is full extension.