FWIW, I never use that little cap on the end of my stainless racking cane. The way it was made made me question how clean I could get it, plus it's something I often simply would forget to use. You do have to be careful not to pull dregs without, but that's a risk even with the cap.
Best thing to do is mark the top with a (real) sharpie. Put it in you fermenter empty, push it all the way to the bottom, then mark of inch/half in/whatever increments from there down. Now you'll have an idea how close it is to the yeast/trub when you can't see the end.
(By "real" sharpie, I mean on of the industrial/king style ones. They seem to hold up to several cleaning cycles. You have to touch them up every so often still, but on the whole they put up with brewery cleaners better than the normal ones.)