Paul, Jobo states on their website that no presoak is neccesary with XTOL. My personal experience has proven this to be true. I use XTOL 1:1 in my Jobo. I generally maintain 100 mls of stock per roll (or two hundred mills mixed developer per roll)as per Kodak's reccomendation. This does limit the volume of rolls that can be processed at one time.(posted 8429 days ago)Kodak's reccomendation of a 100ml stock minimum per roll, and a stock dilution of no higher than 1:1 is thereway of insuring the user against the so called "failure" that some users have reported.
Can you use greater dilutions? Sure. Can you use less than 100 ml of stock per roll? Sure. Will you have to increase development time? Yes. Wll you have consistent results using less than 100 mls of stock per roll? Somewhere there is a threshold where too little stock per roll will create "failure". You will have to run your own tests to find that out.