275 gal tote works well for us for 300 gal sprayer, tank valve in the big lid fits nicely, if using a pump to pump out I would suggest installing an additional vent (unless you have plenty of extra totes around!) It just fills up as we are adding chemical/ams/etc. as long as you have decent water pressure/volume filling to tank.  As for the separation as you asked about, I would think you would be ok diluted down as much as you should be, for most mixes as long as you typically empty it out pretty well, may be a bit different if you stop spraying 50 gal before you run out with some mixes.  Good luck!