Calling trajpar wizz-kids

At 75 degrees, you will not obtain a normal on both ends... technically, that is 180 degrees of phase to obtain that.

What I think you -are- trying to tie the 75 degrees to is the wrap angle around your object, and this is easily enough to accomplish.

the trajpar in this case is a sin/cos function. Technically, a cos() because you want a tangent end (to vertical). Your sign sweep is 180 degrees; your amplitude is whatever makes the project cover 75 degrees around your object. For that you place points at your endpoints and capture a measurement that can be used in your curve sketch relations.

Simple remember a few things... your amplitude is the horizontal distance... while the cos() function will swing from -1 to 1. Add the math to get the appropriate amplitude based on the horizontal measurement (you can do some math here too but it is not directly cylindrical so you have to know the diameter at each endpoint).

Open trajpar features are values from 0-1, however, cos and sin versions require the angle. The angle will be the value from (0 to 1) * 180

Do be careful, however... this is still a projected sine curve and it will not be accurate at any angle other than the projected angle. A wrap will be different as well and will require different math. In order to make is a sine curve along the surface, you will have to use some extra techniques and a different approach.

I do not 100% understand what you are asking.

Is the red line going to be a trajectory for something on the side of the revolve?

Or are you talking about the blue line?