Finally, by the help of professor, I found out how to make all points on the radiant lines to have a random range. The tip is not to calculate the position but to multiple the position numbers by a random number. Because X, Y, Z values are all multiplied by the same number, the new number will keep staying in the radiant lines.

Thus, with some more buttoms added, this mel script is completed. I add a seed bottom which let users able to defined by which random number will the web being created, and also a clear bottom which helps users to white out the scene.

Get spider web mel.

The reference image I finally chose shows a golden spider web. This image has a very rich yellow tone which suggests the time was likely close to sunset.

In my render version, instead of simulating sunset atmosphere, I lighter the yellow color on the light to create a more morning feeling.

Top: the reference image.
Buttom: MAYA render man rendered result.

I tried to animate this web by nCloth.

At the beginning I met a problem that curves couldn't be parented to nCloth surface. However, later I found instead of using parent, use wrap function can make curves attach to the nCloth without any problem. Thus I build the nCloth and it's constrain frame in the script.

Top: playblast of the moving web.

Buttom: final render result.