ideology influence

I was wondering what is actually important for spreading my ideology (or defending against someone elses)?
Is it only my tourism against theirs, or is it my tourism to their culture/their tourism to my culture difference?
For example - Me: 100T, 200C
They: 50T, 300C
So I have a 33% influence, and they 25% so I am able to pressure them with my ideology...

I believe pressure is generated by the difference in the cultural level (Exotic, Popular, Influential, Dominant etc.)

http://forums.civfanatics.com/showthread.php?t=503211

Take a look at the second post.