half power beam width

Someone correct me if i am wrong:
1. find maximum amplitude A[0] on radiation pattern (main lobe peak)
2. go left and right to points, where A[phi] is half power of A[0] (or -6dB).
3. beamwidth is phi_left-phi_right

you can use 20*Log10(x) to convert x amplitude to dB.

which would of course read 3 DB on a power meter or spectrum analyzer face!