[Tom Line]

Are you sure that picture is what you really have? Using the rect->polar conversion results in 0 -> 360 degrees, starting with zero on the right and increasing in the counter clockwise direction.

Regarding the 90 degree shift, yes, you're thinking correctly. Now you just have to figure how to normalize it (add or subtract 90 degrees) and how to bring it back to +/- 180 (hint - use the division with remainder function)