18mm ply backstop

Hi all so i made a pellet stop box the other day and bit a few pellets into it. The back is made out of 18mm ply. So i shot my hw95 into it, all pellets caught lovley same for my hw80 and my bsa ultra. Springers are .177 and ultra is .22. All pellets caught in wood no issues. I have slabs behind just incase. So today i get my brocock commander back from being repaired and he upped power to 11fpe on the dod as she was slightly low. Anyways went to re-zero it put first shot down range couldent see where it hit so went to have a closer look walked to 20mtrs to find a hole clean through the board. The gun is a .177 and shooting bisley magnums. Does this seem abit much power to you guys. Should i go back and get him to test fpe again?
 
Also if the back plate is sloped that can help direct / deflect the pellet/ energy downward
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