I'm just reporting my personal experience. My stuff was a SOLID 10" away to start and I had BIG problems.

I DECIDED go by the old theory of "Keep it all 12" away"-----and my problems went away faster than a fart in a hurricane.

I am using a Bosch sheilded cap. It's been on the ignition system from the very begining. I even tried to sheild the spark plug WIRE initially. You can buy coaxial cable at Radio Shack. It's big enough in diameter that you can slice it lengthwise down the side and pull the wires out of the center. What your left with is the external shielding. It's big enough to wrap around the plug wire. I wrapped it around the plug wire and then cut a few pieces of 3/8" heat shrink tubing and shrinked the sheidling over my plug lead. Had to remove the plug cap first, but it's easy to solder off and then solder it back on.

Didn't make a bit of difference untill I moved everything 18" away.