And a bash entry: 186 <strike>190</strike> chars
---------------------------
for((y=-(r=1,ドルr/2*2);y<=r;y+=2));do for((x=-r;x<=r;++x)); do((x*x+y*y<r*r))&&{((++n));echo -n '*';}||echo -n " ";((x==r))&&echo;done;done;((s=1000,p=n*2*s/r/r,a=p/s,b=p%s));echo "$a.$b"
Run with e.g. `bash py.sh 13`