Jim, there must be enough developer in the tube so that the film will be totally immersed in it when the tube is left standing. I allow an extra centimeter or so over the edge of the film just to be certain it stays under "water" even if some slight movement or shifting of the film might occur.
Ron, the tubes Don built for me are fantastic! I used to own some Darkroom Innovations type tubes but Don's 3-piece design is superior in every way - especially in protecting the film during insertion and most importantly during removal. As per his recommendation, I have now standardised on using a fiberglass mesh screen backing for my negs each time I use the tubes.
My procedure for minimal agitation is one minute initial agitation and then 15 seconds agitation at every 3 minute interval if total time is less than 30 mins or at every 5 minute interval if total time is greater than 30 minutes. My total times are between 2 and 3 times that of my times for continuous but gentle agitation.