I believe the /5 and /6 tach both read 2:1 - meaning that if the cable spins at 1,000 rpm, the tach will read 2,000 rpm.
They're kinda close, usually within a couple hundred RPM. But unless they've been rebuilt or gone through and lubed, you never know without comparing to an accurate tach running at the same time.
As far as adjusting the carbs, there's a lot of leeway so don't sweat it. The range is something like 800 - 1,000 rpm. If it's too low it'll die when you blip the throttle, and too high it generates heat that won't be dissapating quickly. I prefer having them idle around 800, but do what feels right to you.
Wanted: Dead, smashed, crashed or trashed gauges
BMW GAUGE REPAIRS - TACH*SPEEDO*CLOCK*VOLT METER *PODs & LIGHT BOARD*