i "think" from memory the mixture screw on the L6 & L8 differs, L6 is left to weaken and on L8 is left to make richer

this is the process i followed when setting the base idle when i put me MSD chip it, had it bang on the co% of 2.7 with siemens injectors. (L6 ecu)

run engine up to temp
unplugg the iscv (leave engine running) (iscv is on top the throttle body grey plug)
adjust idle on throttle body screw to 900rpm
re connect iscv, and idle should stay stable.
(do this before the cooling fans come on)

Turn ecu screw in 5 turns, this takes it back past zero, then 2.5 turns out.

the screw only has 5 turns of adjustment, so wherever it is set,turning it 5 turns will make it zero. Remember, its a potentiometer, not a normal screw, so it only adjusts a certain amount, no more or no less, even if you turn it 100 times, turning it back 5 times will still take it to zero.

2.5 takes you to the mid pount,which is a good starting point