YOU SAID:

I did pretty decent on this test

INTO JAPANESE

私はこのテストにはかなりまともな

BACK INTO ENGLISH

I am pretty decent for this test

INTO JAPANESE

私はこのテストのかなりまともな

BACK INTO ENGLISH

I am pretty decent of this test

INTO JAPANESE

私はこの試験にかなりうまかった

BACK INTO ENGLISH

I was pretty good at this exam

INTO JAPANESE

私はこの試験にかなりうまくいた

BACK INTO ENGLISH

I was pretty good at this exam