90 degree angle seems a bit extreme. You want to see the monitors side by side, and pretty dead on straight. When you look at monitors from an angle, you start to see false colors and such. Plus, you don't want to be turning your head too much to one side or the other.
As long as they are eye level, and only as slight angles, you just want to see each one striaght on as much as possible. Distance depends on personal taste and your eye sight, but roughly 20-40 inches is recommended.
Here are OSHA guildelines.