Where do you get the 800mm from?
I know about T-72M1s being penetrated at close to 4000m with the rod leaving the back of the turret in relatively good shape.
What is your possible protection level of a T-72M1 frontally + the back of the turret?
T-72M1 armor is known to be :
Turret: 380(490) mm
Glacis : 400 (490) mm
Data are from NII Stali itself.
So, front+back KE armor is probably close to 450-480mm.
Note, T-72M1 is more or less comparable to later 70x USSR T-72A.
Comparing T-72M1 armor and T-90 armor is like comparing M1 and M1A2SEP armor.
in reality, there are several variants of T-72M1, and these data are for most protected one. I dont know which variant was penetrated at 4000m. Still, given what fairly modern round with nominal penetration close to 500mm at 4000m range was used it is very possible what the round penetrated both front and and back turret armor.
P.S. 800-850mm is based on estimation supported by Vasiliy Fofanov. More known Paul Lakowsky estimation gives somewhat higher figure, but general consensus what his estimations are fare too generous for all tanks (he provided figures for most modern tank including western tanks).