The Equation for a Standings Increase:
10 - (10 - (Current_Standings)) x (1 - (%modification) / 100)


If you complete a Storyline mission for a Faction and it increases your Faction Standing by X,
then the faction increase to that factions allies is 0.5X
and decreases your standing to that Faction's enemies by 0.5X and to its allies by 0.1X

The Equation for a Standings Decrease:
(10 + (Current_Standings)) X (1 - (%modification) / 100) - 10


How the Standing Increase from the skill "Connections" is calculated.
Effective Standing = Unadjusted Standing + (10 - Unadjusted Standing) * 4% * Skill Level.