mid-distance -how to work out the Add? (more accurately than "half the near Add")

[maryjane]

Hi - a quick question about intermediate-distance eyeglass lenses.

My prescription for distance lenses is
(R) Sph -1.75, Cyl -1.25 Axis 165,
(L) Sph -2.00, Cyl -0.75 Axis 035,
and my near Add is +1.25 for both eyes.

I have bought distance glasses and reading glasses.
My usual distance for reading books is ~ 35-40cm (14-16").
The reading glasses are fine for reading, but things start to blur fast
at greater distances than 18 inches.
My usual distance for reading a computer screen is ~ 55-60cm (22-24").
So I'm thinking of getting some intermediate-distance lenses.

The 'usual' intermediate Add is half the near Add.
But is it possible to be more accurate than this, given the actual distances involved?

(I also have to say - I'm not sure why the usual Intermediate correction is the arithmetic mean of the values for Distance and Near, given that the Sph and Cyl figures are in dioptres, where 1 dioptre is 1/metre - so I would have guessed the harmonic mean would be a better mean, but that's another discussion!)

Thanks in advance!

Mary