I use a mixer that I built, but I just dump in as much pickling lime as I feel like. The dosed liquid should be a saturated solution, so I saturate it! So if there is undissolved pickling lime, that's no big deal. Of course I only dose the LIQUID and not the slushy undissolved stuff at the bottom of the mixer. I'm sure I waste a tiny amount, but in general as fresh water is added to the mixer, it dissolves more PL until it's saturated!
My mixer is like 4.5 feet tall, so my mixing pump is at the bottom and fresh RO/DI water is pumped in with a dosing pump near the bottom, and saturated limewater/kalkwasser is taken from the top (4 feet above where the mixing and RODI input is).
Just be sure not to dose the slush and only the liquid and you'll be fine!
V