This is what I've found works for me, but there's bound to be simpler methods.
I stand with my feet at the same height as the top of the cauldron, but back a little from it, to throw items in and then stir them with the spade several times until I get the bang. Obviously make sure your crosshairs are in the middle of the cauldron when you drop the materials in.
I then hop down to lower than the cauldron but beside it to retrieve most of the results (some pop out at my feet, but very few).
I'm not an alchemist atm, but this was working for me a couple of days ago.