Guess. Don't be too crazy.
I believe that you roughly want to figure out a 5-band to fit an item into, then slot it into almost any of the levels in that band. (that is 1-5, 6-10, 11-15, 16-20, 21-25, 26-30)
Compare to other items, and all that, but you'll see even items that look almost identical that are off-level by 1 (or worse)