If you can hit the card every time then start at the recognised shooting distance of 10 metres. (Shots missing the target are to be avoided for obvious reasons)
6 yards was the old NARPA discipline and is still shot at clubs that do not have 10 metre facilities using the later NSRA 6 yard target. A supply of 6 yard targets are handy for practise in your garage where range space is limited
This 6 yard target was designed by the NSRA so that most shooters would get similar scores at the 2 disciplines.
PS Don't start at 20 yards as this is very unforgiving for new shooters