How to define spring constant of a pile for design of a pile foundation?

Dear Sir,

I am designing a pile foundation in staad using the plate modeling and defining the pile supports as springs. How should i calculate the spring constant.

My vertical pile capacity is 150MT and allowable deflection of pile head is 12mm. Is it 150/0.012=12500. Or should i multiply my capacity with factor of safety and calculate as (3*150)/0.012=37500??

Please clarify.