German WW1/2 Tank, Anti Tank guns and more of 7,92mm - 21 cm

They had plans and limited conversion of 12,2 cm s.F.H. 396 (r) (M-30) to 12,8 cm.
Aside that they came to the conclusion that production of own 12,2 cm Sprenggranate and Hohlladungsgranate (which is identical in performance to 12,8 cm so 15 cm pen).

Spoiler






how do you get the performance about 8.8cm aphedsfs, by calculating or guessing?

I just put it in the calculator, taking it as a simple aphe.