The conversion is no rocket science.

For relative angles simply add a minus to revert the direction, this works in both directions.

30° geographical becomes -30° cartesian and vice versa.

For absolute angles it becomes a bit more work.

To revert the angle use 360° - ANGLE.

To compensate the the rotation add 90°.

This leads to the formula:

(360° - ANGLE + 90°) MOD 360

Where MOD is for the modulus operation, it gives the division rest and limits the result to be between 0 and 360.

I spreadsheet syntax it should be something like this:

=MOD(360 - A1 + 90; 360)

Where

*A1* would be the cell reference with the angle to convert.

This formula works for conversions from geographical to cartesian and vice versa too.

Sadly the LibreCAD command line parser doesn't support modulo operator for direct input yet.