What method or devices do you typically use for balancing planes these days

two small pices of tape. One under each wing where i put my fingers on for initial CG. then i adjust on triming flights to what ever feels good. If im going to change to a lighter battery i calculate how much i need to move the battery to get the same CG

Do you balance mid-wing pattern planes upright or inverted like low wings?

It doesn't really matter if one way is stable and the other way is unstable, so long as the stab is roughly 0deg at the tipping point then it's accuracy is good plus or minus a few mm.

Most will argue that determining exactly where the CG is ranks far lower than actually having it in the best location determined by flight testing. It's a different story if you're swapping different weight batteries and need to reproduce an exact location for consistency. For that, I'd get it close, flight test and mark the individual battery's location.

Once you know the rough CG location, then using a calculator to figure out any changes takes all the guess work and heavy(?) lifting out of it.

I find use of my index fingers as the most practical method.

BJR has the right method. Put the CG in a safe spot and tune based on flight.