I recently came across a radio shack CB/HAM mag mount antenna (#21-972).
I want to trim it down for 2m use. What would be length I would look for?
The steel whip is 39inches and it's total height with base is 46 inches.
I think it may be between 19.5 and 19.75 inches. Could someone confirm my
measurement?
Thanks.

I have not look at that antenna, but when it is advertised as a cb/ham
antenna it is usually for the ham 10 meter band or the CB. At the bottom of
the antenna is a loading coil. To use it on 2 meters you will have to
remover the coil in the bottom of the antenna. If it is a 2 meter ham
antenna that is about 40 inches long it will be a 5/8 wavelenght antenna .
You need to look at another antenna for 2 meters if you want a 1/4 wave whip
without any loading or matching coils. Then it would be close to 19 to 20
inches long.