The Debens do have a limited amount of MOA per turn. From memory 7.2 MOA. My Steyr with sports match risers and atp 61's is running at something like 6.5 MOA... even at 795 fps...
How many clicks do you actually use? It could be scope height. Helen runs a high AJP rail and ATP 61's and she's got something like 4.5 MOA on her walther running at the same speed, on a Deben.
I'd be tempted not to shim if you can avoid it as it can stress the tube and the inner parallax mechanism. You can find with more shims the side wheel starts to stiffen. With the APT's you might be able to jack it up more at the back to get it closer to optical or lower on the turret... mine jumps when it runs out of travel, so I avoid going near it.
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