How to calculate lightning protection Risk Level for the project?

The Risk here is defined in the standard IEC 62305 as "the probable average annual loss in a structure and in a service due to lightning flashes"

Risk factor depends on 

1-The annual number of lightning flashes influencing the structure and the service

 2-The probability of damage by one of the influencing lightning flashes;

 3- The mean amount of consequential loss.

Now for calculating Risk you have to perform Risk Assesment as per IEC 62305- part-2

The decision to protect a structure or a service against lightning, as well as the selection of protection measures, shall be performed according to IEC 62305-1. The following procedure shall be applied: –

identification of the object to be protected and its characteristics; –

identification of all the types of loss in the object and the relevant corresponding risk R (R1 to R4); –

evaluation of risk R for each type of loss (R1 to R4);

– evaluation of need of protection, by comparison of risk R1, R2 and R3 for a structure (R′2 for a service) with the tolerable risk RT; –

evaluation of cost effectiveness of protection by comparison of the costs of total loss with and without protection measures. In this case, the assessment of components of risk R4 for a structure (R′4 for a service) shall be performed.  

lighting =1cubic feet =8 watt