I use Marshalls' dyes. A day before I am going to spot, I take a drop of the dye and put it in a place of my palette. Put one or two drops of water let dry overnight. It will dry up and form a very thin layer of dye. Bottom is the thinnest and goes darker as it goes up the wall of the palette.

Next day, I take my very small spotting brush and barely wet it with water. I pick up the right shade of the thin die from dried up dye I made yesterday. Brush lightly on spare piece of paper and make sure the density is right. More importantly, I make sure the brush is not too wet. For me, having the brush just barely moist but mostly dry works the best. Also, if too dry, bristles separate so it needs to be a bit wetter than that.

Spot, spot, spot. Look. Repeat if necessary. I typically wait for 5 minutes after all done and review my work.

Letting the dye dry first in this fashion allows me to pickup very minute amount of the material easily, accurately, and repeatably. I can exercise far more control than diluting it on the spot.

If I really mess it up, I wash the print again and dry, then repeat.