If both the original combatants managed to whittle away each other's strength equally, then it might be that there would be 200 Starfleet and 800 Dominion ships left - in which case 600 Klingons might be needed. This would probably be true if all ships fought duels rather than group actions.
If they damaged each other in constant ratio, though, there could be 200 Starfleet but just 400 Dominion ships left, in which case just 200 Klingons would turn the tide. This would be true only if the individual ships of Starfleet were twice as good as the individual ships of the Dominion.
However, in melee battles like that, a combatant gaining a slight upper hand would typically be able to concentrate fire to quickly eliminate one opposing ship, leaving a greater ratio of ships to overpower the next victim, and again a greater ratio to deal with the next, and the next - until the initial underdog would be losing ships at an exponential rate, and the top dog would suffer virtually no losses. In which case 200 Starfleet ships left might indicate 1,100 Dominion ones left, and some 900 Klingons would be needed to make a difference.
It appears that the ships did not fight duels; that Starfleet ships in average indeed were better than the smallest Dominion ships, although probably inferior to the bigger ones; and that a melee did take place, but without chance of significant concentration of fire, of "destruction in detail" tactics or other such factors. So an arbitrary mixture of the above models can be postulated, and the number of Klingon ships chosen exactly as one pleases.