If you are making a simultaneous ZC motion, I would guess that the linear axis feedrate is going to be 'boss' and will determine how long the motion requires to complete. The rotary axis is internally synchronized via interpolation, to end the feed motion at exactly the same time as the Z motion completes.
For a sole rotary axis feedrate, you need to calculate the circumference of the part, to determine how many degrees per minute it takes to travel an equivalent linear distance. There may even be a setting in your control parameters to enter the diameter that the C axis is working on.
To determine the combined vector feedrate of both the linear and rotary combined motion as you mill the helix, you need to find the length of the hypotenuse of a right triangle:
The base of the triangle is the lead of the helix, or how far along Z it takes to complete a full revolution of the feature.
The height of the triangle is the circumference of the part, generally assumed to be at the largest diameter.
From those two sides, you can compute the length of the hypotenuse, which is the path the tool is on. Subsequently, you can arbitrarily choose how fast you want the tool to travel along this hypotenuse.
Now, you reverse engineer the Z component of the 'hypotenusal feedrate' using what....COS of the helix angle (between hypotenuse and the Z base). So the linear feedrate of the Z should always be less than the actual desired feedrate along the helix.
Since a helix like a twist drill is quite a gentle slope, there is not going to be a huge difference between the Z component and the hypotenusal net feedrate. Its barely worth the trouble to calculate the difference since its like only a few percent on the feedrate override.