Most steampunk or Victorian monocle's & pendants you can find are actually optometrists & or ophthalmologist trial lenses.
my 'prescription' ended up as #-9 & #-11 snellen or -3.50 & -4.50 diopters
trial lenses with the tabs / handles , trial lenses without the tabs / handles , & bare trial lenses in a modded frame
older lenses were biconcave )( while modern lenses are concavoconvex ))
Prior to the 1880's the focal length of corrective lenses was in Snellen's inch standard
After the 1890's the focal length referred to inverse fractions of a meter in diopters
The meter at 39.37 inches is rounded up to 40 to convert between snellen & diopters
If the maximum distance you can see is 10 inches, a little more than 25 cm,
you're very near sighted & you would need a -4.00 diopter or #-10 snell lens.