If you’re interested in the distance the lens must be extended Δ forward its infinity position to focus at a lens-to-subject distance s,
Δ = sf/(s - f) – f
Example: f = 90mm, s = 3 meters = 3000mm
Δ = 3000*90/(3000 – 90) – 90 = 2.78mm
You’d extend the lens 2.78mm forward of its infinity position to focus lens-to-subject distance s = 3 meters.