query about clipping threshold calculation for papr reduction in ofdm.

can any body tel me how to calculate the Clipping threshol for clipping technique to reduce the PAPR of OFDM signal?

is there any specific formula to calculate it?

what parameters we need to assign or consider to calculate the clipping level?

in many programms from forum i got clipping level as 0.4, but i am confuse about its selction criteria,

please help me at the earliest!!!!!