A comment by kesler12 asked me to provide some support for my claim that in actual battle units usually break after losing a minority of their strength (the context was that simulated combat where men do not fear for their lives tends to have much higher casualty rates and much more resolute defences).
I couldn't find the original summarising article or report that I would have preferred to supply and I sure won't even try to assemble the dozens if not hundreds of separate tiny bits that form the general picture, but I did find an old study with a statistic analysis that seems relevant:
Robert L. Helmbold, RAND Corporation, 1971
He tried in typical (then still new) operational research style to develop a formula for calculating when a force would 'break' and retreat. His hypotheses had to be tested, and he used a database of 1080 historical battles, and the author produced these diagrams:
As you can see, the winning defenders suffered up to 20% casualties in about 90% of the battles and 20-25% casualties in another about 5% of the battles. Meanwhile, the attacks - which faltered obviously - suffered less than 40% casualties in more than 90% of the battles.
Next, the same from 612 battles won by the attackers. Again, about 90% of the losing defenders suffered "only" up to about 40% casualties.Keep in mind that it's a rule of thumb that usually more casualties occur during pursuit than before the defence broke!
These were historical battles, many of which were rather unlike modern combat engagements, but human nature largely stays the same, I suppose. Either way, the results are so very striking in support of the assumption that defending forces yield after suffering less than 50% casualties that this should be accepted as true IMO.
What I remember from the old summarising article that I keep not finding (I suspect it's in Infantry Magazine 1990's, Armor magazine 1980's or 1990's or in Military Review somewhere, or maybe it's some RAND report.) is:
Many defences break after 10-20% casualties over a time span of hours or days. The most resolute "stubborn" defending forces suffered up to 40% casualties before retiring.
Other sources support more assumptions:
Only encircled forces or forces that couldn't flee (such as Japanese island defenders) went to 80% casualties and beyond without routing. Attacks stall after similar (lower) casualty rates as defences break, and they stall the earlier the more "veteran" and less "youthful" the units are. "Veteran" units tend to be more stubborn (less fragile) defenders (and less vigorous attackers), and so are forces with a higher share of married men (who tend to be less prone to give up).