There's only one plane of focus.
Things further away or closer to the camera are out of focus.
However, objects close to that plane of focus are not as blurry as objects far away from it. What you can do is to define a threshold of blurriness for yourself and thus derive an area around the plane of focus to be acceptable for you as "close enough to being in focus".
Even with the formula, you still need to figure out what your threshold is, calculate the result and apply it. It's not practical to know the formula.
What is practical however are the parameters of the formula, which leads to the question of "how to maximize background blur for subject separation in the foreground".
- Use the most open aperture possible. That is the smallest f-number.
In your case, f2.8.
- Use the longest focal length, in your case 70mm.
- Get as close to the subject as possible.
- In theory, a larger sensor also helps, but this is usually impossible
to change unless you brought two different bodies.
Take a look at this question with excellent answers:
What exactly determines depth of field?
Which one of these lenses [85mm f/1.8 | 24-70mm f/2.8 | 35mm f/1.8] would be a good choice for this kind of shot?
The 85 will no doubt blow out the background very much. Maybe a bit
too much. I think you included the trumpet player intentionally. A
dancer alone can look awkward in an image due to the frozen motion.
Adding the trumpet player in the background tells a lot more about
the situation. It's also a rather long lens, which squeezes the
elements into a tighter space. But a dancer is everything but
something that can live in a tight space. He/she moves around, making
it hard for you to get both people into the frame. Don't get me
wrong, if you can get everything into the frame, the 85 will produce
stunning images. But with the situation of a moving subject in a
confined space, it won't be easy. Like a portrait session with the
subject jumping around, basically.
35 mm allows you to get more in the frame. This allows for more
variation in the composition (think both behind each other, side by
side, ...). It would be less of a "portrait of the dancer" approach
but more of a "two people picture, with emphasis on the dancer". If
your goal is to show the interaction of both, the 35 would be a
better choice over the 85.
With the zoom, the separation is reduced, but you can adjust to the
movement of the dancer best. I do not own such a lens, but I guess it
should provide enough separation to draw the attention of the viewer
towards the focused dancer. If your goal is to blur the trumpet
player to the degree that he/she just appears as a mush color in the
background, this lens is not the best choice. But again, I don't
think this is your goal as you could simply compose the frame without
the trumpet player in it. I think I'd go with this lens. It will
produce a lot of keepers as it has the flexibility of an adjustable
focal length allowing your to "dance along" with your subject.
The aperture advantage of the primes allows you to use shorter shutter speeds, which in turn freeze the motion better. It depends on the light situation, how far you can push your iso and how fast the movement is, but it can be helpful to get a few more stops of light that you can invest in shorter shutter speeds.
On the other hand, a slow shutter speed with a 2nd curtain flash can do wonders to an image containing motion. But that's a different story.