Feels like there should be a simple rule, doesn't it?
You'd be working with sums of reciprocals though, so no simple addition
rule. Easy enough to derive it, though:

In the example, 'N' is pretty close to 2D.
39.25" (1 meter) divided by 20" = 1.9625D, if it gives you the effect of
seeing the object at infinity.

You need 39.25/28 = 1.4018D *total*, so -0.56D "add".
Close enough for almost anyone, -0.50D.

If you tried to use the 8" difference directly in a formula, you'd get
something like 39.25/8 = 5D, definitely NOT what you want.

(All that supposes you have or want to use no compensation, or focusing
power of your own lens. Post-cataract surgery, e.g.)

Dave

It is better than a rule of thumb. Gauss's imaging formula gives
accurate results. For a lens of focal lengtyh F or power (diopters)
D=1/F, it is

1/Object_distance + 1/Image_distance = 1/F or D.

Read Wikipedia.

Bill

No rule of thumb would make it easy to add reciprocals. That's why diopters
were invented, cause it's easy to add diopters. It's also why people who
work with lenses eventually learn the diopter value of several common focal
lengths.

For a human with no accommodation, corrected precisely for infinity, a +2.00
lens focuses pretty precisely at 20 inches. +2.00D=1/2m=20"

+1.75D=1/1.75m=.57m=22"

+1.50D=1/1.5m=.67m=26"

+1.25D=1/1.25m=.8m=31"

Obviously, quarter-diopter steps are much smaller up close than when further
away. That's the difficulty.

When the steps don't line up exactly (26" vs 28") common sense usually
dictates whether we round up or down. Less is usually better than more, and
in bright light, depth-of-field usually gives us at least +/-0.25D of
leeway.

-MT