The cooling off period is described by UKVI as follows:
If the application is for entry clearance or switching from another leave category within the UK, a 12-month cooling off period applies.
The applicant must not have been in the UK with entry clearance or leave to remain as a Tier 2 migrant at any time during the 12 months immediately before the date of the application, unless one of the exceptions below applies.
Where the applicant left the UK before their Tier 2 leave expired, the 12-month period can start earlier than the date their leave expired but only if they can provide evidence of having not been in the UK for a period of 12 months immediately before that date. Acceptable evidence may include, but is not limited to:
travel tickets or boarding cards or passes (in paper or electronic form), but only if the applicant’s sponsor submitted a Sponsorship Management System (SMS) report at the time, confirming that their employment in the UK had ended
exit or entry stamps in the applicant’s passport which confirms that they were not in the UK
a letter from the applicant’s overseas employer confirming the date they started or restarted work overseas, after returning from the UK
any other evidence that shows the applicant was not in the UK
So you should ensure that your employer informs UKVI promptly about the end of your employment in the UK, and that you save any other evidence you have of having left the UK.