I have committed to scratch mixing as many chemicals as I logically can. As Kirk noted, selenium powder is reputed to be extremely dangerous to handle, so I will stick with premixed. Awhile back I costed out the bulk chemicals to make TF 3 (or maybe 4, can't remember now) vs. buying TF4 or 5, and unless I calculated wrongly, the savings were so minimal that I decided to stick with premixed. And I can't find a lith developer formula that works as reliably as premixed, but everything else is scratch made.