My main question is how long should the flash channel be from the bottom of the slot to the inside cone?
I like to get it as short as possible, around .010” to .015”. Some would call it almost nothing. I use a #2 center drill with a .078” diameter nose (a little large for many) that I work from the powder side to cut the chamfer. I have been doing this since the 1980s and quite honestly any concerns of a flash in the pan went away when I was shown the technique. I do have a few center drills with the nose ground down to .060”, but just don’t feel I get the performance I want with the smaller size.
I have guns using liners with slots, hex holes, and flush (Chambers White Lightening) and get the same performance from all with the short straight, a chamfer on the powder side and a .078” diameter hole.
Building a gun right now and debating if I want to install a hex liner or a White Lightening purely for the pleasing look.
As far as wearing out the threads by removing the liner, call that an urban legend. You don’t hear about the danger of wearing out all the other threads on guns (and other metal mechanical devices). Wonder why? A properly sized and lubed liner isn’t going to wear out threads in your lifetime.