Jess, for the Loft top, it says 21" from shoulder to hem.
If you are bad at measuring yourself, I would take a top you already own and measure 21" from shoulder to hem. Then put a piece of masking tape there. Then you can try on the top and see where the hem on the Loft shirt will hit.
Loft often has sales, so if you see something you like and it's not on sale, wait a while, because you should be able to get it at better than full price.
I don't have a particular top from BR in mind. I just mentioned BR because I know that they usually list the length of items on their website. I hope that if you go there, the sales associate has a measuring tape! Sometimes SAs are clueless, unfortunately! But the SAs should know what they have that might have the right length of hem for you, so it's definitely worth a try. This personal style thing is a lot of trial and error, believe me.
I was never good at measuring either, but I got better at it. You just have to start somewhere.