In the testing for the double twill I found that the selvedge was a bit tight, making the dreaded "smile" curve across the web. I was using an 8-end satin on 12 ends on each side of the web. I tweaked the file to make the selvedge a 12-end satin, a looser interlacement, and tried it out. It seems to be working better, and the fell is level. For those who care about such things, here's a picture of the new selvedge.
This makes me happy.