Regarding accuracy ...
I absolutely love my 5" Cut Rite Charm Maker ruler. It's made especially for cutting 5" charms, and is simple by design (no excessive markings, which eliminates the possibility of mis-measuring). It does have markings for 2-1/2" squares, which is great for that purpose, but you can also use the intersection of those lines to fussy cut a 5" charm. Just center up the design on the intersection, and cut your square.
Here's a link to it on the MSQC site. It's very reasonably priced (under $7). If you watch, they occasionally have free shipping for their "Daily Deal".
Other places probably sell it, too.
https://www.missouriquiltco.com/shop...te-charm-maker
(No affiliation with MSQC or Quilter's Paradise other than being a happy customer.)