Tourism works a lot like culture. It is accumulated over time as a total counter. If your tourism is +8, that means your "BASE" tourism is 8 per turn. (So after 10 turns you will have 80 total tourism). The base tourism is modified by several modifiers with each civ. Open borders for example has a 25% modifier. So if you have open boarders with Civ A but not with Civ B. You will generate 10 tourism with Civ A per turn and 8 tourism with Civ B. (So after 10 turns you will have 100 tourism with Civ A and 80 tourism with Civ B). Its a running total (just like culture is).
Your influence level is determined by the total amount of tourism over the course of the entire game compared to the total culture generated over the entire game of the other Civ. If you have generated 1000 total tourism to Civ A and Civ A has generated 10000 total culture then your influence is 10% (and thus at the exotic level). This is seperate for every civ since each civ has different tourism amounts you've put into them (due to modifiers) and they have generated different amounts of total culture.
The unhappiness from different ideologies uses the 6 influence levels of culture. Points are awarded as follows
Unknown (0% - 10%) = 0 Points
Exotic (10% - 30%) = 1 Point
Familiar (30% - 60%) = 2 Points
Popular (60% - 100%) = 3 Points
Influential (100% - 200%) = 4 Points
Dominating (200%+) = 5 Points.
To find ideological pressure, you take your influence level and compare it to theirs. If they are Familiar and You are Exotic, they will influence 1 point of ideological pressure on you. Also If I recall, if the world congress has voted on a world ideology, that ideology generates an additional 2 points of pressure.
As for how much for each level of pressure (Content, Dissidents, Civil Resistance, and Revolutionary Wave), I'm not certain how much pressure points it takes from different ideologies.
Basically what it boils down to is that if you even dont plan on playing towards a culture game, you can't just ignore tourism completely. It doesn't take much to get to exotic with every civ in the game and lowering ideological pressure by 1. Your only other option to avoid the happiness penalty is to just not take an ideology at all or swap to the world ideology.