Blasts -> Promyelocytes -> Myelocytes -> Metamyelocytes -> Bands -> Segmented neutrophils (This is the natural progression from earliest progenitor cell to the final mature state)
To me, "Left shifted" means I am seeing early granulocyte precursors (e.g. blasts, promyelocytes, myelocytes, metamyelocytes) out in the peripheral blood.
When I call something "left shifted" I like to qualify what I am seeing. I may say, "left-shifted leukocytosis, including occasional myelocytes and metamyelocytes". Usually, a differential cell count will be provided either by the lab tech or pathologist. To me, there's no such thing as a cut-off percentage for reporting a left-shift. You have to interpret this in the appropriate clinical context. Sure, if you look at a gazillion peripheral smears, you will undoubtedly find a rare, circulating blast cell in an otherwise normal individual's peripheral blood. If you find that you're seeing a significant number of immature granulocytes on the differential, then you know that you're dealing with a left-shift.
Leukocytosis has nothing to do with whether there is a left-shift or not. Leukocytosis simply means elevated WBC. Often times, when you have a leukocytosis, you will probably notice at least a mild left shift. Sometimes, you may not. What if you had a CBC that showed a WBC of 17K with a differential of 92% lymphocytes? Obviously, this patient has a leukocytosis and we know that this is primarily due to the increased numbers of lymphs (more specifically qualified as a lymphocytosis). There was no mention of left shift in that example.