Hi Pete!
There is an easy way to create~view a magnifier rectangle that follows your cursor and is easy to toggle on & off. You can set its size and magnification, and the shortcut keys to activate~toggle it. It works well for close in work. (Please see Fig.1 below.)
And it functions while You do a selected area screen shot; no more trips to the image editor to fine tune!
Fig.1 - Screen shot of the magnifier using the parameters I spec'd.
1) To set up this magnifier go to "System Preferences" > "Accessibility"
2) Select the "Zoom" section.
3) Fine tune things using the "Zoom style:" > "Options..." button
