The Nsra Rules list Target Dimensions as below (http://www.nsra.co.uk/index.php/downloads/rules) :

1989 SERIES - LONG RANGE
Scoring Ring (Inward gauging - see Rule 5.1.6)
Value 100 yards 50 yards
X Ring 12.40 4.09
10 26.48 9.03
9 57.32 23.66
8 88.16 38.29
7 119.00 52.92
6 149.84 67.55
5 180.68 82.18
4 211.52 96.81
3 242.36
2 273.20
Aiming Mark
205.55 102.78

Also, not quite what you wanted to know, but this link contains useful advice : http://www.hamptonrifleclub.org.uk/consider.htm.

HTH,

Vic Thompson.