Hi Niels, thanks for the link - very intriguing.
I haven't really thought about depth vs diameter - obviously the greater the number of depth X diameter, the harder it will be to bore. Essentially, I just want to progress to boring holes with a lathe. I've avoided it for a long time, but it's a very important thing to be able to do confidently. My main interest of course is boring cylinders for model engines, but having got some confidence with it, I expect it will open up many more possibilities.