Quote Originally Posted by Trevor Crone View Post
As a matter of course when using Paterson System 4 Universal tanks I use a min. of 350 ml for 35mm film and 600ml for 120 film.

This is to allow for possible upward movement of the reel even with the retaining clip in place, and to prevent uneven edge development caused by 'frothing' of the developer.
Something to consider as an alternative is having some lengths of PVC tubing to take up the slack at the top. An eBay seller shipped me a pen packed in a PVC tube once, and by nice coincidence, the tube closely matched the shaft diameter of my Paterson tanks, so I chopped it up into short spacers, and, voila, no more rising reels.

I wind up using 600ml for 120 film anyhow because I use XTol 1:2 with two rolls on a reel with a minimum of 100ml stock per roll. I still have a couple of rolls with a thin edge where the reel rode up the shaft prior to my using PVC spacers.