You first have to estimate approximately where the sun will rise, this can be done trying a few waypoints along your flightpath.
You then choose one waypoint before the sunrise and one after the sunrise.
You get four times out of that :
- SRA : sunrise at point A
- ETA : estimated time over A
- SRB : surnrise over point B
- ETB : estimated time over B
You can plot this times on a graph, draw the SR line (connect the two dots), draw the ET line.
The lines should intersect, and you can read the time you will see the sun rise.
If you use a chart to do the plotting, the intersection will show the position of the sunrise.
To work properly the points have to be "close", 10° of longitude works well on an Atlancic crossing.
You could also compute the intersection using some simple math, e.g; find the intersection of two lines each defined by two points.
See the picture here :
http://tinyurl.com/yfxrf2b