急问: 关于做 indepent contrator at a Hi-tech company

Hi, I want to be an independent contractor at a hi-tech company in the silicon valley. 请教:

1) since I need to pay health insurance myself and I don't have any benefit such as 401K/ESPP, so how much benefit loss/insurance cost that should be reflected in my pay? Any standard or formula I can refer to?

2) I will be contracted through a agency company, normally does the agency charge one-time fee from my client company or they will also continue sharing my hourly pay? If it's latter, do you know usually how much percentage they will take?

Thanks a lot in advance!