After 20 years, finally got my act together and figured out a system to purchase the right scope covers the first time.
Purchase dial calipers. I like the analog ones since the digital readout will eventually fail (same with tire pressure gauges):
https://smile.amazon.com/gp/product/B01C6WHQ92/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Identify affordable and reliable scope covers:
https://smile.amazon.com/gp/product/B078S3SSBZ/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
Measure inside diameter of the objective. Do not measure the outside, or they will be too big!
If analog calipers, convert to MM:
https://www.stefanelli.eng.br/en/simulator-virtual-dial-caliper-thousandth-inch/
https://www.rapidtables.com/convert/length/inch-to-mm.html
If your objective falls between two sizes, order the smaller diameter cover. Example: If you have a 51.5mm objective (between 50.5mm - 53mm), choose the 50.5mm cover.