I develop FP4+ in ID-11 at 1:3; the times you mention - 20 mins at 20 degs C. @125 ASA - are correct. That's what I use and I'm very happy with the results I get; very fine grain, loads of detail and smooth tonality. The advantages of the 1:3 dilution are increased tolerance of errors; 120 seconds represents a 10% error, whereas 10% of 11 minutes is 66 seconds. Mis-timed development becomes less important at 1:3 dilution, as do temperature errors; you've more time to warm or cool a waterbath. Of course, it's also more economical and your developer stock goes a lot further.