When lenses are close together, their diopters add. Diopters are $m^{-1}$, the inverse of the focal length. So, when I wanted glasses for bench work, typical distance 50 cm, I asked my optometrist to write me a prescription for 2 diopters more than my distance prescription. They work great!

So, to bring the focus from infinity to 40 cm, add 2.5 diopters to whatever focuses on infinity.

When lenses are close together, their diopters add. Diopters are $m^{-1}$, the inverse of the focal length. So, when I wanted glasses for bench work, typical distance 50 cm, I asked my optometrist to write me a prescription for 2 diopters more than my distance prescription. They work great!

To bring the focus from infinity to 40 cm, add 2.5 diopters to whatever focuses on infinity.