In Burrard's load chart he shows the 2½" 28ga standard as 1 3/4dram or DE with 9/16oz shot, with recommended size of #7 & smaller (British size). He also gives a "Low Velocity" load of 1½ Dr or equiv, with 3/4oz shot, recommended for size 5 or 6.
The 5 & 6 are approx equivelent to our #6 & #7. For sizes equivelent to our #7½ or smaller he does not recommend the low velocity load.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra