I have to add my experience about diluting the big kits --
The undiluted original concentrates GO bad in the original bottles after opening in about 6 months. I have seen this with developer AND the fixer part of blix. You have to transfer them to new, squeezable or small bottles if you do this...
HOWEVER, I have found that both developer and blix keep very very well after mixed to the final dilution! This has been somewhat surprising to me. It is probably partly because the diluted solutions are very easy to keep completely airtight in 100% full or squeezed bottles. My experiences with PET and HDPE bottles, air squeezed out, are both very good. About year for both developer and blix, at least for now.