Hi, 

Which kind of experiment are you talking about?

Best regards.

I am doing trust calculation between sensor nodes in sensor network.

for a trust calculation,

 For examples 40 nodes environment experiment, is a 70% of selfish nodes environment possible? 

Yes why not. It depends on the scenario. I recommend to you our paper related to trust calculation and variation in different scenarios. Our contributions is based on VANET, but it possible to adapt them for WSN.

- A distributed advanced analytical trust model for VANETs. IEEE. IEEE GLOBECOM'2012, Dec 2012, Anaheim, California, United States. IEEE Press, pp.219-224, 2012 (you can get off version here : https://hal-upec-upem.archives-ouvertes.fr/hal-00724667/file/TrustModel4VANET-GC12.pdf)

If you are focusing on the trust management and incentive mechanisms you can read this paper :

- A Job Market Signaling Scheme for Incentive and Trust Management in Vehicular Ad Hoc Networks. IEEE Transactions on Vehicular Technology, 2015. (you can get pdf version here: https://hal-upec-upem.archives-ouvertes.fr/hal-01070598/file/IncentiveTrustManagementVANET.pdf) 

If you are dealing with detection and monitoring of selfish nodes you can read this paper where the contribution focus on the time overhearing optimization. 

muDog: Smart monitoring mechanism for wireless sensor networks based on IEEE 802.15.4 MAC. IEEE ICC'2011, Jun 2011, Kyoto, Japan. pp.1 - 6, 2011

https://hal-upec-upem.archives-ouvertes.fr/hal-00620375/file/Monitoring-15-4.pdf

Thank you sir

surely i will read it.

is there any formula to calculate trust?

Yes. You find the detail in these papers.