I highly recommend the MYTOL - I use it a lot for push processing or for big enlargements. It also keeps well if it is denied Oxygen. I had some stock solution I mixed up in February (6 mo old) and used it last week. I had it in one of those "accordian" plastic bottles and so there was no air in it. It worked perfectly. (It had yellowed a little) I have not tried mixing it as a concentrate but that sounds like a good idea. I bet the concentrate decays less than the working mixture over time.