Usually when I encountered distortion it was from my pad coming down too hard onto plate when picking up the ink. Also make sure that you pad is hitting dead center on your plate image.
Residue on the pad is not that big of a deal when doing multiple imprints. It is very common and as long as you do multiple prints hitting the same exact area it shouldn't matter too much.
As for imprint size in relation to the pen...it's mostly trial and error however if you can find someone else that offers the same pen and does imprinting they normally list the imprint size in the details.
Thin pens are a pain. My only other suggestion is to offer imprinting on pens that you are comfortable with. I only offer pen styles designed with large imprint areas so our customers can get more bang for their buck.
Take care
Jon
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