Implemented feature by default in FW (IIRC since 1.1.43):
Use triggernum=1 for 4,6cyl, =2 for 8cyl
Use samlenum=15 for 4cyl, =10 for 6cyl, =7 for 8cyl
It is good upto 6-7krpm, but if you use hi-reving engine you probably need to reduce samplenum for good readings other sensors.

Obsolete (it was proposal, implemented feature is more advanced):
My book says:
MAP sampling 1ms (best) or 2ms (good too).
Each sample should added to summ, and at trigger event we have clear MAP value: MAP=summ/count.
We have ADC sample rate 1ms in firmware, i have made every 2nd ADC reading for MAP (exept time while WBO sampling). And average readings on trigger to trigger period(s).
Two config variables: minimum samples for averaging (3-5) and trigger periods (1 for 4cyl and 2 for 6cyl)
Last two ADC channels dropped, knock ADC never used (firmware uses knock chip advanced mode) and 2nd ego (for boost target or NBO), both never used in my setups.