I see what you mean then, so Fredericksburg and Cold Harbor would fit the mold in that case where the attacker suffered disproportionate loss.What I mean is that if there's a battle with 10,000 engaged on each side and one side suffers 5,000 while the other suffers 4,000, and another battle where one side suffers 2,000 while the other suffers 7,000, then in both cases there are 9,000 casualties at the end of the day. Both battles have been as bloody.
Now, looking specifically at Chickamauga:
Casualties and losses ∼ 60,000 ∼ 65,000 16,170
4,757 captured or missing
1,468 captured or missing
So of 125,000 men on the field there are 28,399 men KIA/WIA. This is 23%.
Borodino sees 250,000 troops involved in the fighting, which conveniently is double the number at Chickamauga.
There were 52,000 Russian troops reported as dead, wounded or missing in the battle, but of these 8,000 subsequently returned to their units and 1,000 were prisoners; this implies 43,000 KIA/WIA.
The French returns for one day of battle (the 7th) give 6,562 dead and 21,450 wounded, totalling to 29,000 KIA/WIA.
In total this comes to 72,000 KIA/WIA, discounting the French losses on the 5th (which is part of the same fighting).
French casualties on the 5th were on the order of 4,000-5,000.
In combination this is about 75,000, or 30%. The "buttress" or the number of Russian MIA who could be removed without affecting the "bloodier than Chickamauga" calculation is about 17,000.
Your Borodino example of total loss is therefore 28%, within a few tenths of a percentage as Chickamauga depending on sources used for combatants present at both battles. This still accords with my contention about losses in mass formations. The French artillery created a charnel house at Borodino whereas CSA guns were much less effective in the heavy woodland of N. Georgia.