"Interested. Be carefull of this one. If i were to use my left side before i place my foot that would be CBMP and i would be stepping outside my partner."
It depends on when the left side achieves an advance. If it is before the start of the step in question (before the feet have passed to end the previous step), then yes this would be a step into CBMP because it is then stepping in a direction relative to the pre-existing condition at the start of the step.
However, achieving a left side lead after the step begins, but before the step is placed does not result in CBMP.
Remember that CBMP is defined as a position relative to the standing foot, not as the result of CBM. It is named for the resemblance to the result of CBM, but it is not defined as the result of CBM because that is not how it is achieved.