So a momentum of +1 decreases loses by 10%, a momentum of -1 increases loses by 10%?
The only problem there is that the momentum hasn't figured into the side that won. You would still want momentum to alter which side wins too. It would also make sense for only one side to have momentum at a time - or that negative momentum is just the same as saying the opponents have that amount of positive momentum.
I suppose you could change it so that the losing side has momentum*10% of their ships escape, i.e. if they had good momentum but they still lose it's like they got out of the fight before they got slaughtered.
And that the winner side suffers loses, reduced by a momentum*10%
So the formulas would be (assuming momentum goes from 0 to 10):
Total number of ships lost by loser = (No Of Ships Committed * 10 - (momentum))/10
Total number of ships lost by winner = (No of Ships lost by Loser * Loser Ship Strength*(10-momentum))/Winner Ship Strength*10
For example (again momentum still not factored into who wins or loses)
Narn with +7 momentum vs Centauri
Narn Commit 10% of there ships.
Centauri Commit 10% of there Ships
Narn Engagement Score =(2000*5)/10 = 1000
Centauri Engagement Score =(1500*8)/10 = 1200 Winner
Narn loses = (200 * (10-7))/10 = 60 ships
Centauri loses = (60 * 5*10)/8*10 = 37.5 = 37 ships
EA +4 vs Vorlons
EA Commit 10% of there ships.
Vorlons Commit 10% of there Ships
EA Engagement Score =(2000*6)/10 = 1200 Winner
Vorlon Engagement Score =(500*20)/10 = 1000
Vorlons loses = (50 * (10-0))/10 = 50 ships
EA loses = (50 * 20 * (10-4) )/6*10 = 100 ships
By having each faction having a momentum value, you could then potentially have things where you have a three way war, and thanks to victories against two sides vs a third, when those two side fight they both have postive momentum.
Other ideas this allows - scouting missions which if the player completes successfully, the players side know how many ships the enemy will commit, and so commits enough to win (more ships means higher engagement score).
*edit*
Thinking about momentum factoring in to the engagement score. You could either have it that momentum increase the effective tech level of ships in calculating the score, or makes a factions ships count for more:
eg:
EngagementScore = ((Ship Strength+momentum) * Number of Ships)/Percentage of Fleet commited
or
EngagementScore = (Ship Strength * (Number of Ships + (Number of Ships * momentum)/10))/Percentage of Fleet commited
e.g.
Narn with +7 momentum vs Centauri
Narn Engagement Score =(2000*(5+7))/10 = 2400 Winner
Centauri Engagement Score =(1500*8)/10 = 1200
or
Narn Engagement Score =((2000+(2000*7)/10)*5)/10 = 1700 Winner
Centauri Engagement Score =(1500*8)/10 = 1200
The Raiders still won't be able to win a battle with the vorlons even with a momentum of +10, but at least wouldn't ever lose any ships in the battle.
You could total up all the engagement scores to get an overall status of the campaign at each stage.
*/edit*
10% of forces in battle for all races is fine, but I suggest moving upto to 25% when the losing team has 20% of ships left (from start total).
I still like the idea of a battle of the line style affect, where when a faction is down to a % of their starting fleet they commit all their ships in a last ditch defence.