When is it good time to negotiate contract rate with agent?

Hi, ALL

  After interview with compnay, agent called, hiring company will ofter  contract postion:  $45/hour. 

I know the hiring company will give agent $70/hour. What's best rate can we negotiate?  Can we nogotiate $60 or $55?  When is good time to nogotiate? Is it better to wait agent officail offer by email then nogotiate?  Or we can nogotiate now, but just worry, if nogotiate now, agent  maybe give other people offer that who didn't ask  rate increase. 

 

Thank you in advance!