While it wont give you an exact number, you can get a sense of how plus or minus a move is by this method:
In training mode, set the dummy to all guard, hit them with a move, pause during block stun, set the dummy to jump, un-pause and hold up. The dummy will jump immediately after block stun and you will jump immediately after recovery. The further behind the dummy you are, the more minus the move is. The more ahead of the dummy you are, the more plus the move is.
-TexasTim-