I'm 60 and nearsighted, currently about -6, and have been
getting less nearsighted for the last 10 years or so after
topping out at about -7.5.  Anyway, I'm now using my
2-year-old reading glasses as my desktop computer glasses,
so it's that time again.

Up to now I've worn progressives as distance and
general-purpose glasses, and separate single-vision glasses
for my desktop computer and another set for serious reading.
So that's three so far.  But now I'm considering getting a
laptop, which I assume would fit in there between the
desktop and reading glasses, but possibly one pair of
glasses might work for two of those three (I doubt it).

Going into this round of lens replacement, I would like to
get an independent source of information as to the
appropriate add for various close-in working distances, just
as a check on what the optician tells me.  Since I have very
little accomodation left, it's important that I do this
right.

In addition, in case I continue to get less nearsighted, I'd
like to see if through careful selection this time I might
be able to "stage" the three single-vision glasses so that
they just shift in function from reading, to laptop, to
desktop, so that I only have to replace the reading glasses
as time goes on (well, and the progressives of course).  The
left eye has been .75 less nearsighted than the right for
about 15 years now, and if that continues, then this might
work.

At first I thought I could just use the diopter to focal
length relationship to calculate this information, but then
I wondered whether there might be things that make that
formula less valid.  Such as perhaps the fact that the
glasses lens is maybe an inch in front of the eyeball, or
the fact that the lenses are spherical instead of parabolic.
And I have this vague recollection that when I asked a
version of this question years ago (about computer glasses),
the optician said that the amount of add could vary somewhat
depending on the strength of the distance prescription.  The
implication was that in actual practice the add for a
specific working distance might be different for a very
nearsighted person than for a moderately nearsighted one,
for whatever reason.

Well, I guess what I'm looking for is a table, or tables, of
distance-versus-add that eyecare professionals use in actual
practice to get the best results for people wearing glasses.
If there is such a thing.

As an aside, I've also thought about under-correcting on the
progressives this time by .25, just so they would last
longer.  Any thoughts on that?

Thanks for any help.