14.30Dominant ♭9 contains a diminished chord on its 3rd

General rule:

For dominant 7♭9:

take the diminished seventh chord built on the 3rd of the dominant.

Example:

G7♭9

3rd = B

so RH B°7.

E7♭9

3rd = G♯

RH G♯°7.

A7♭9

3rd = C♯

RH C♯°7.

Very useful piano shortcut.