I don't understand spline meshes rolls

(please use simple words (my English is very bad because I am french :sweat_smile: :fr: ))

Hello,
I don’t understand what is the functioning of the start and end roll of the spline meshes :
what number I need for rotate of how many degrees of rotation?

Why if the mesh is smaller the needed number is smaller?

can you give me a Mathematical formula for convert from degrees please?

thanks for your future anwser!