The only thing that is needs to be understood re Total Depravity is that man's heart is desperately wicked, and therefore ALL man's four faculties therein are corrupt. All = Total. Easy to understand.
Desperately wicked does not mean the same thing as totally depraved. All faculties of sinners are imperfect. That does not account for the faculties of those who have not yet sinned. Nor does it get you logically to the conclusion that sinners who are imperfect people cannot do any good with their imperfect faculties.