I suspect no more than 1/3 of Southerners supported the slaveowners' rebellion in 1861, that 1/3 consisting largely of the plantation elite, the gullible, the perpetually ignorant and the ultra-provincial who thought Yankees were of a different nation.
Yes, a very intense one-third seems to be the general historical rule.
You are probably quite correct, up till the fighting started. Then, of course, over 90% of southerners, reasonably enough, fought to defend their homes.
But the group that intrigued to gin up the conflicts that led to secession, the "fire-eaters," were a quite small minority.