you can't really mark the position, cos there's more than one turn of the pot available. if you do adjust it (and i wouldn't unless you have to), then carefully count how many turns you make, and be carefult not to reach the end and damage it.

an erratic idle can also be caused by a too rich or too lean mixture (as well as a range of other faults), not necessarily just a dodgy iscv.

best to get it properly setup (by a pro with a wideband lambda sensor)