+1 for above. (Jefrayg).. but I am sorry to admit that I use "spit" it works well.
The other RULE... put the bottle of spot-tone way the hell on the other side of the room!! (This is the most important thing I learned in college)
I used a piece of 5x7" glossy paper as my pallet.
Foam core works well to. When I needed more spot tone, I inverted the closed bottle and too it with the 000 brush from the cap.
It is easier to add spot-tone than take away!!! Start with a clean print... sometimes if you get that "down" you may not need to spot.