600,000 used to be the accepted number of CW killed, but you are correct that this number has been rising in recent years as more complete scholarship becomes available.
The 1860 census counted 31.4 million Americans.
To have 750,000 killed out of 31 million is a tremendously high amount. If females were one half of the population, now you are looking at 750,000 out of 15 million men, or 5% of the male population. Of fighting age population, the figure goes much higher.
In the South, many of these men were the head of a farming household. To lose this man would have impoverished an entire family.
Some communities in the South lost 30% of their men.