Distance between same shot and two focal lengths

Lets say I want to grab a shot of a bridge.  I use a T3i and have my kit lens set at 17mm and the bridge fills the entire shot.  Now I want to get the same shot using a 70-200mm lens set at 70mm.  How far would I have to walk backwards to get the bridge to fill the shot?  Would it be 50', 100' etc? Just looking for a rough estimate not an exact figure.  Is there a formula for this?

Thanks for the help.