Source:
German Steel Armour Piercing Projectiles and Theory of Penetration (BIOS Final Report No. 1343 ADM 213/951) (19.9~ MB ZIP)

Specifically Page 27 [Hard Copy] (Image of Page), where it says:

Tierberg, the head Krupp designer until recently, was clearly a man of the old school who hated innovations. He was very autocratic and simply could not conceive of any design being better than his own. The only formula he ever used was:

for normal attack.

He then relied on his curves for penetration. The penetration curves appeared sound in principle as the hardness of the plates used varied with thickness and therefore a single, or relatively simple, formula to embrace penetration for all values of t/d was impossible. There would, inevitably be discontinuities at various arbitrary points where owing to increase in thickness of plate, the tensile strength was lowered in steps.

If we assume that the formula uses the same symbols as the others in the BIOS report, they are:

V: Striking Velocity (m/sec)
W: Shell Mass (kg)
D: Shell Diameter (mm)
T: Armor Thickness Penetrated (mm)
C: Constant (varies slightly for different calibers)

A rearranged equation to find for T alone via OpenAI is: