Gerry writes:

Trying to figure out the link between magnification and focal length.
I need glasses to be able to read and see what I''m doing clearly on
my lathe but the 1.50 safety glasses I have force me to get too close
to my work. Looking for something with a focal length of maybe 22" so
I can stand at a comfortable position. Does that mean a stronger lense?

The focal length in diopters (D) is the inverse of the focal length in
meters. So 1.50D readers have a focal length of 1/1.50 = 0.67 m = 26
inches. Objects that far away will appear to be at infinity so if you are
presbyopic and otherwise corrected to focus at infinity that is the working
distance. If you're otherwise nearsighted or farsighted then you need to
add that power into your calculations. Sounds like you must start out a
bit nearsighted if 1.50D readers force you to get too close.