Focal length, framing, aperture and dof calculation question

Supposed that I want to get a head-and-shoulder portrait photo and I shoot using the new Olympus 75mm 1.8 wide open, at distance 2m from the object and get DoF of 4cm.

If i want to use 50mm lens instead:

1. How much closer should I move to cover the same frame as previously? (I don't care about the background/ perspective distortion, I just want to cover the same head-and-shoulder)

2. What's my aperture to get that same DoF?

Can you please present the formula as well?

Thanks.