I have a rifle that must use the screw-driver vent due to a high thread and an inability to open up the hole sufficiently for the White Lightening liner. I was able to measure the depth of the flash channel and noted it was long, long enough to use a diamond stone in a dremel tool, to dish the vent liner around the hole to just below the depth of the screw slot- not a cone per-say, but dished. This allows the powder to come up to the side of the vent as in Dan's photo and ignition is as perfect as it can get, with the cheap, poorly made, screw slotted vent.
The problem was the length of the flash channel - once rectified, it works just fine, even though the vent hole is way too high above the pan. Before firing, I tilt the rifle to the left, so the priming comes over against the side of the barrel and thus the vent. Ignition is now pretty good. Forget about buggering up the screw slot- doesn't matter. When the vent needs to come out due to wear, use an easy-out, just as you would with a Whitelightening liner. Mine has a 1/16" vent hole and it doesn't need to be any bigger. Use a 1/16" drill to open your up if it already isn't that size.