Dont' think u can over cure using the UV method, wheather it be Sun light, uv nail boxes, or the B9 units. All that is really required is complete surface cure. Complete penetrating cure can't hurt.
If u sand, file, or drill a cured print you may expose uncured resin, so redo the curing after any type of suface work.
As far as enough cure then cure all your prints underwater in the B9 unit for 3 to 6 min depending on size/thickness. If u have a flat charm or similar then cure on one side and flip it over and do the same again.
I leave the bases on my ring prints so they stand up vertically for good even exposure to the UV.
The only bad results from curing that I have experienced came from over curing with heat. Results were cracking of the prints. This only happened to me with B9 black and I really pushed the limits with it just to see what would happen (2.5 to 3 hrs at 350F). Never had a print crack using heat if ramping was done properly (recommended is 2 deg per min or 120 degrees F per hr. to 340f and hold for 1hr.).
As I have stated many times in the past, if u are having failed prints, post pictures with sprues and button. At least other casters have somewhere to start with trying to help.
Dance like nobody's watching!