If you have the optimization license, you can let CATIA adjust the length of the other axis until the perimeter of the ellipse is exactly what you want.
Otherwise, it requires a calculator or alot of trial and error.
Is it possible to constrain curve lenght? (I need only one quarter of the ellipse)
I wish to find better solution. Now I "solve" this problem vith VBA, and as I already explained computer need cca 2 min to calculate exact value (error should be less than 0.00001)
You stated "I have it." That is the "IT" in my post. I assumed that you had derived the equation referring to your previous post after rereading you meant that you have the optimization license.
If you provide the application and you current model it will be much easier to help.