I'd be tempted to just go with each side per combat round, takes 10% of the other side's point value.
So assuming a chart:
(snip)
Of course you would have to take into account speed of the two formations, scouting capabilities (i.e. Side B spots Side A first and chooses not to fight), consumables (i.e. ammunition), and other fun.
If you're doing something that simple, make it a random number - (2d6+3)%, perhaps. A quick spreadsheet to do that, with your same 1000 vs 800, gave me the following results:
P1 wins round 12 with 575 left
P1 wins round 11 with 650 left
P1 wins round 13 with 553 left
P1 wins round 15 with 423 left
P1 wins round 13 with 550 left
P1 wins round 12 with 595 left
P1 wins round 13 with 542 left
P1 wins round 13 with 600 left
P1 wins round 12 with 614 left
P1 wins round 13 with 563 left
So overall it's in the same ballpark, but it's got some variance.