Since your studio, I assume, is set up to use the 85mm lens on 35mm then you should understand that a 210mm lens is going to see a bit wider, something like a 60mm lens might see on your 35mm camera.
Just understand that with the 210 you might need to move the camera position forward a fair bit to keep the camera from seeing past the edges of your background. You might want to consider a bit longer lens say 300ish if moving forward would put you in a bind or you simply want to keep the same camera position.
When I do the math I use the short edge of any film as my reference vs. the focal length. In your case I get a 24mm:85mm ratio. For 4x5 film 4"=~100mm which is essentially 4x24 so then we go 4x85 which suggests a lens that focuses on the subject when it is 340mm from the film.
This isn't an exact suggestion for focal length on LF because, unlike 35mm cameras where the lens is solidly attached and focusing doesn't change the angle of view appreciably, focusing on a close subject with a LF camera means moving the lens furthar away from the film which will narrow the cameras angle of view.
A 300mm(ish) focal length lens would be a reasonable guess to get focused on a portrait subject with the lens somewhere close to 340mm from the film.