In the first year that the MPAA applies, it is only pension input amounts for money purchase arrangements that occur after the date of the trigger event that are measured against the MPAA.
So if, for example, in the 2018/19 PIP a trigger event occurs on 1 August 2018:
For a DB scheme the amount tested against the MPAA is based on a factor of the number of days after the tax year starts. If the MPAA was triggered on day 165 of the year, then the amount tested against the MPAA is 200/365.