Explain map at idle
I assume that means you want the highest map possible at idle.
At the same time, isn't vacuum a pressure difference? So does thy mean you want the map reading as low as possible? Meaning a greater pressure difference between the engine and atmosphere?
The less air (hence fuel) you need the more efficient you
are. That's about as good a measure as you can get, for
idle tweaking.
Of course the same also holds for part throttle loads like
towing, if you could get the load steady enough to be
useful in tuning you could maximize fuel economy by
chasing it to minimum there too.

