Does anyone have an idea of the life of the working solution? I currently use TF-4, but I'm down to my last bit of it, and didn't know if I should just buy some more or just make some TF-3. One of the main things I like about TF-4 is that the working solution lasts for quite some time, so I never have to problem of using it to capacity. I've read TF-3 is good for about 20 rolls of film, but how long will it last if you don't reach #20 very soon? Thanks for any insight you all can provide!