ooooo... I Love this topic... A L O T.

Technology/Magic and Culture Advancement Rate = The relative Number of major outside influences.

All of these things will advance to a certain degree, because sentient beings (like humans) push themselves to delve into the next great mystery and move things forward to better their way of life. Barring any outside influences, the relative rate and level of achievement will usually progress in a forward manner with sporadic 'Leaps' forward. (by leaps, I mean when someone, somewhere hits on that Eureka Idea and moves their field forward... hint: E=MC²)

Outside Influences however, happen and are usually detrimental to a societies progress. The biggest one of these is probably war and being conquered. Many civilizations have met the end of their days simply by being subdued into the history books (Aztecs come to mind). Then of course.... Plagues, Massive Crop failures, Draughts, Earthquakes, Volcanoes, Cats and Dogs..... you get the picture. So, that is what you can apply from real world perspective.

Add in the bit about fantasy and well, the equation is no longer an equation but more or less a gigantic web of possiblities.

Religion is a big one in a fantasy world I think. Especially in a place where the Gods do walk among mortals and have direct influence on events that shape the world. Beings of Supreme power and unlimited resources (some anyways) can have a direct bearing on culture and technology.

Its hard to think what kind of legends and myths you could have in a world chocked full of dragons, wyverns, dire toads and leprechauns. Simply the word Legend and Myth become synonymous with fact.

I actually had a point when I started, but I've had to many errands in between writing this post, so I think I'll quit while I'm ahead