When my rail screws came loose last Spring, I found I could get nearly 4" of windage at 40 yards. I never thought to center the scope with it. I just tightened it down parallel to the frame
You can also shim the back of that rail up, under rail at those screws, to elevate the rear to get you all the use of the scope's MOA dialing potential that you desire to keep the scope at optical center as well.