I made the experience that putting the pistons on or near TDC really helps a lot, like stated on that Norton site.. It takes at least twice as much time for the oil to drain down. I guess the hydrostatic pressure from the oil in the big ends helps to reduce seeping through the pump and maybe helps the ball valve to close properly, but I'm just guessing here. Whatever the reason, it does help with both my bikes. I always wondered why it sometimes took more than a month for the oil to drain down, and sometimes just 2 weeks, so I tried different positions and watched the wet-sumping progressing.
BSA_54A10, I totally agree with you. It is just a matter of regular use. With my bikes, it is sufficient to ride them once a month, and the drain plug can stay untouched.
Imho after all it's these little imperfections that make the charme of owning and maintaining an old vehicle. I mean, (almost) anyone can get on a new japanese bike, press the start button and take off, but where's the "get-your-hands-dirty fun"?
Cheers!