There is a reference "Route Surveys and Design" by Hickerson. My 4th edition, chapter4 on "Compound and Reverse Curves" had details that may explain your questions. I can explain the second part about superelevations for small and large radius. Superelevations are based on sliding friction due to centrifugal forces being greater for smaller radius than for larger radius at a given velocity-- basic physics. The first part may have some relation to the type of curves ( circular, parabolic, spirals....) but that's just a guess on my part. If you study the reference book which involves trigonometry, I believe you'll have your answer to the first part.

Thank you,

my question if I use 2/3 eL at BC it gonna result in 1.6 % at BC which is lower than 2%

May be because my super elevation rate is not high thats why it come like that
superelvation for larger radius = 2.4%
superelvation for smaller radius = 3.4%

i dont have option to change curves,. i have to stick with existing road layout

One thing the Green Book rarely does is advise on what to do when an existing alignment doesn't meet normal conditions.

Don't forget to check the grade at your PRC. You'll have no cross slope there, so make sure you have some longitudinal grade to move the water.

Also, the run-on and run-out rates are normally the same. It should be a straight line from normal crown to full bank, and your reverse curve transitions won't be the same length on both sides of the PRC because the superelevations are different.

What's your design speed?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/