How do you do this

Flitz
Valve grinding compound
wet emory paper
grinding wheel
blacksmith

I started with flitz and cant confirm I made any headway

I`d spin it in a drill and touch it with a piece of very fine - 1500+ grit emery and measure for any change. The directions that come with the dies caution against removeing more then .001" You`ll also have to measure it to start to have a referance to go with. It won`t take much so go slow.

Are you sure you need more tension? Have you tried rotating the case 180* and hittting it once more? This will add little to the tension.

Another option is to call Lee. They can make you a custom one or send one that is a different size.