First, if you use only a checkmark to indicate a checkmark-within-a-circle, I would likely applaud your choice to simplify where appropriate. The explanation of this checkmark usage should go on the Transcriber’s Notes Page. It would probably be appropriate to identify the checkmark itself on the Special Symbols Page, but the use of only a checkmark where print shows a checkmark inside of (or on top of) a circle, is a braille formatting choice and so belongs on the Transcriber’s Notes Page. (BF2016 §3.1.1’s second paragraph and RUEB §3.28)
–Kyle