The size of the bullseye in relation to distance is always considered in field target. The size to distance difficulty guidelines in FT is called " the Troyer scale".http://www.aafta.org/resources.html#/The link above shows three things that may be of interest to you. Have a look.RiflePistolMaxHunter
Not illegal, just needs to be no further out than 15 to 18 yards.