If you have a simultaneous move combining a rotary with a linear axis move, then I would assume that the linear feedrate is 'boss' because both motions must begin and end at the same moment. IOW, the feedrate along a ZC vector will be the same as a feedrate along a single axis Z vector movement.
For a single C axis rotation, there you might have to consult the manufacturer's instructions for C axis feedrate programming.
In a machine like a Haas, there is a setting that one can make where the OD of the part is input for the job at hand. So using this value, the controller can determine (automatically) what the feedrate in degrees would be for a rotary axis, based on the last linear feedrate in mode. So a Haas is nice in that regard, that we don't have to be mixing linear ipm commands with dpm commands.
Perhaps that information will help you figure out what to do. Not all controllers handle this in as neat a manner, so you'd have to read the manual to see what to do.