I don't have a coily so not sure what leg length my coil is at. I usually lineup my aromamizer atty coil(s) to where the bottom of the coil(s) sits right at the top of the bottom row of airflow holes to help get more airflow to the bottom of the coil(s).
For coil ID I went with 4mm. I prefer my coil(s) to sit around 1-2mm away from the airflow holes and 4mm put me right about there.
Right now I'm running a tri-core fused Clapton with 26GA cores and 36GA fuse. Funny thing is that those picks are right after I built it. I'm now vaping that same build at 52W.
Gotta admit the coil is producing good flavor despite not being even close to my favorite coil build. Gonna build a 29GA .3x.1 framed staple for it this weekend. Can't wait because I've always found that framed staple to be more flavorful than the tri-core. And vape at a lower wattage too.