Everyone repeats the advice to angle. Fewer people can say what angle, and past what point it stops helping.
The short version: effective thickness scales with the inverse cosine of the impact angle. At thirty degrees off perpendicular you are getting roughly fifteen percent more, which sounds better than it plays. The large gains do not start until you are past sixty.
The part that gets left out is that this only describes whether a shell penetrates. It says nothing about the shells that were going to overmatch you regardless, and against those the angle you chose is irrelevant.
So the rule is narrower than it sounds. Against comparable calibre, angle hard. Against something that overmatches you, the only useful variable is not being seen.