Until you have to flush twice to actually get rid of everything. I'm a bigger fan of the dual flow toilets. Low flow for liquids, higher flow for solids. But that does cost money, and require you to own your own home.

There are also adjustable floats/drain mechanisms which you can adjust so you use 'just enough water'. Making your tank only hold a little over one gallon is a bad idea since you would have to flush twice and two flushes of 1.4/1.6 gallons per flush would be a waste instead of adjusting and 'just' using 2 gallons up front.

It is as much common sense and practicality as 'what is the minimum'.

And there is always the maxim repeated in California back during their decade long drought from the 70s -- if it's yellow; let it mellow. If it's brown; flush it down.

Goofy but they were spray painting their yards green back then too...

Lots of things to keep in mind!

one way to reclaim is to recycle, if you don't use the sprayer in your kitchen, knock out the hole in the sink for it, run a pipe down into a 5 gallon bucket ( or if you already a grey water tank connect it to that) connect up a funnel top side, then when you are running water waiting for it to warm up, have the faucet over the funnel. Reuse the water in your garden, in your toilet, or if you have a grey water recycle system, you already know where that's being reused.