If you have over 200mm of penetration, shoot it in the lower plate, or the upper half of the upper plate. I know it shows as red on the hitskin, but the hitskin is lying to you. Tanks like the IS-3 will have no issue punching straight through, and if you load gold, it's nearly impossible to bounce. Basically, the only spot you need to avoid is the lower half of the upper plate.
The other place to shoot it would be the "missile launchers" since there is a tall, easily pennable cupola behind them which is also relatively easy to hit. The turret cheeks are also pretty easy to pen.
Also, even though the mantlet was stealth-buffed (you used to be able to pen that too), you can also shoot exactly where the gun is, that's only ~150mm of effective armor
Edited by Posit1ve_, 13 August 2019 - 06:21 PM.