Working out angles... I'm doing something wrong here

I finally have the land blob area derived from the cross product! 8-P
I have the page looking at the fan area size and comparing this to real world continent areas.
Tonight I just want to start building a second triangle fan. I already have randomly found a point index in the land plob shape. I need to know the preceding and the following point to get an angle and derive a new center point halfway around an arc between those two...