I'm with Mike. You can't get any simpler than using the "Arrival
Altitude" navbox. It takes into account all available parameters
(distance, Mc, bugs, ballast, wind), and is corrected for reserve
altitude. This requires you have reasonable polar info entered for the
ship you're flying. This feature is perfect for first forays into xc
flying. You can use this to "hop" from one airfield to the next. A
quick glance at the navbox will show whether you can make the current
waypoint from your current position. Negative value = climb, 0 or
higher = good to go.