First of all the probiblity of the round hitting is worked out, if it does the angle between the surface and the incoming round is calculated, then you could use the De Marre formula which is
(velocity ^2 * Mass * (cosine angle) ^(2/n))/ diameter ^3 = C(thickness of plate/diameter of shell)^n where n and C are constants.
However for sub caliber rounds at high angles +30 degrees the n in the (2/n) part of the equation is reduced to 1.11, the represent s the tendancies of these round to break up at steep angles. This was a change that was added in the most recent patch as certain people had noticed the penetration of sub claiber rounds was too high at extream angles.
Now to use the equation you will need the values for constants. You can use the German 75 L48 as a base on which all rounds are standardised. In which case the value for C is 4.25 approximatly.
The value for n is around 1.4-1.5 the higher value the flatter the nose is. For APDS the values are 5.6 for C and 1.37 for n reducing to 1.11 for angles greater than 30 degrees.
BTW this is only how I would do it and it can get alot more complicated.
There are some other factors smaller shells can be destroyed by larger plates that simply will not move out of the way even though the penetration should be greater than the plates thickness, this is the so called shatter gap. Also the t/d ratio can have an effect on this process.