The measurement is based on 'Squad Leader Survey' with a change in scale from '1 to 3' to '1 to 5'.1
a 'balanced' scenario yields no wider split than 60/40.2
Source
1 Joe and Mike Suchar, 1981 Nov.-Dec., Squad Leader Survey, the Avalon Hill General, Vol 18 Issue 4, 16-18
2 Jon Mishcon, 1979 Nov.-Dec., East Front Asides, the Avalon Hill General , Vol 16 Issue 4, 15-18