According to my references underside roundel was of A type and was painted there until 06.06.1940 when the colour of undersides was changed. Then they stopped applying of the roundel on undersides. On the 1st August 1940 the A roundel was introduced on undersides again. It was allowed to apply a roundel of 76,2 cm - 30 inches in the diameter, 114,3cm - 45 inches, 127cm - 50 inches but also 63,5cm - 25 inches, 101,6cm - 40 inches. The proportion of the roundel was: N - the max diameter of the sign (25,30, 40, 45, 50 inches) , the diameter of the white B=3/5N and the diameter of the red C=1/5N.
As Stona said , no position was initially specified. However here you are a couple of locations of the roundel centre from the wing tip.
a roundel 50" - 182,9cm - 72"
a roundel 40" - 175,3cm - 69"
a roundel 30" - 114,3cm - 45" or 53,3cm - 21'
a roundel 25" - 45,7cm - 18"