I just found this site :
update : the website content of that link has changed. I found other website where you can download the parabola calculator pc app (freeware) here :
where we can draw a parabolic curve with variation parameters.
So I tried to google some of the parabolic modifiers and find a matching curve for each of them.
The black dot in the axis line is the location of focus point (our barebulb strobe).
As you can see in the picture, the Broncolor, PLM and Rime Lite are quite parabolic while the Westcott Zeppelin and Profoto umbrella are not.
I have the Paul Buff PLM, Rime Lite 120cm and Aurora Tera D (same as Westcott Zeppelin).
Though the Aurora is not true parabolic, I like its light output.
Any other parabolic modifiers I should check?