[ErikCumps]

No hard game data is used for the influence calculation, it is not as such recorded in the savegame.
At least not that I know. If it was in there I would have used it.

Instead it's a very simple heuristic where for each hex a blue and red influence value is calculated and the highest value wins.

Each blue and red unit contributes to the total blue and red influence value, with modifiers for distance, unit readiness
and relative height difference.

The frontline is basically the edge between the blue and red influence zones.

As I said it is caluclated based on data in the savegame:
the battle does not have to be concluded, nor do you need savegames for all the turns of the battle.