Spoiler - Industry S3E7: Adler IRR Mental Math

Spoiler Alert 
 

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When running through a board deck in the 50th minute of Industry S3E7, an erroneous PE multiple is spotted. Bill Adler then notes within a matter of seconds what the IRR impact of correcting the multiple error would be.
 

I’m curious to know how you’d go about mentally calculating a shift in IRR as Adler does here. Or alternatively:

1) This is all just random financial jargon which doesn’t actually stack up in reality (think three glasses inglorious bastards scene); or

2) The cop out answer would be there is a sensitivity table in front of him which he’s reading off, but regardless would like to know if there’s a simple trustworthy way of working it out mentally