I agree that 90% seems to be the upper end, but 50% is the extreme low end and is a dated figure and the other sources seem to show this. Notably that quote you cite was written in the early 80s, and according to most those sources, wars are getting more deadly for civilians and less deadly for combatants (drones and other remote killing methods?).

UNICEF et al cover this at length. https://www.unicef.org/graca/patterns.htm

https://www.iraqbodycount.org

Edit: Intersting discussion here, suggesting much of the difference comes down to how you measure and when you stop measuring. The longer you measure, the worse it is for civilians and in particular, women. https://files.prio.org/Publication_files/Prio/Armed%20Confli....