Highest ABV to charge still

[rickyaifd]

It is my understanding that you should never charge the still with any mash, wash,or low wines, more than 40% ABV or 80 proof. Am I correct in my understanding? I've seen various commentary talking about 40%, 30%, but it isn't clear if that is 40 proof which is 20% ABV or 30 proof which is only 15% ABV.
I just want to make clear in my understanding that the total alcohol by volume should be no more than 40% ABV going into your still at any one time