According to site :"The design of the SEAMEO flag was chosen at the 6th SEAMEO Council Conference (11-14 January 1971). The adopted design has the SEAMEO emblem in the centre with a light or sky blue (azure) background. The dimensions of the flag are 1.42 metres by 2.20 metres and the diameter of the emblem is 0.85 metres. The then Khmer Republic (Cambodia) was accepted as Member Country during the Sixth SEAMEO Council Conference held in Saigon, 1971 with immediate effect. However, the Council agreed not to change the SEAMEO emblem through the addition of another "branch" to the existing seven "branches". It was to be understood and accepted that the seven "branches" of the SEAMEO emblem were purely aesthetic and did not indicate the number of members of the Organization."
Alteration on the Logo
By 1998, the seven founding members have grown to include all ten countries in Southeast Asia. The following countries, aside from the seven, now constitute the membership of SEAMEO: Cambodia, Brunei and Myanmar. Three more rays are added to the existing seven rays of the logo to reflect the aforementioned development concerning SEAMEO Membership.
The flag graphic is under the header "Original SEAMEO Flag," implying that changes had been made. Given that the flag is a logo on a plain field, I find it unlikely they would have changed the logo but kept the flag. The proportion figures cited--142 x 220 cm--come out to 216 x 334.8 px, which leads me to believe they are rounded off and should be 2:3. Likewise, the diameter of the logo--85 cm--translates to almost exactly 3/5 height.