I hated spot-tone dye because I could never mix "just right". .
One trick with spotone is that you don't mix at all. As you go along, you just keep diluting at the edge of the beaker or whatever you're putting your drops on. And another trick is that you don't throw anything away, just let the remainder dry and re-wet it for the next time.
This way one single drop of spotone will easily last a year. I can't remember the last time I added another drop.
I've got both, and use both. Depends on the paper I printed on, whether or not I toned the print, how much spotting needs to be done, the destiny of the print, and what I feel like doing that day. I tend to be faster with the pens.
I simply get a small white watercolor palette, fill the individual indentations with water, cut up the Peerless dye sheets into small pieces and place them in the water until they bleed out and remove them. Once the water evaporates, it is easy to wet a brush and use them. The dyes last forever like this and it is a simple task to mix different colors as I often need.
Sumi ink works well for spotting fb and is fairly easy to use.
Get a couple of sticks, one cool black, one warm and you'll be set for life and several reincarnations as well. Of course you can blend the inks as needed.
I like a pure white Corelle dish from Walmart, 'Winter Frost' as a pallet, it's thin enough to put on a light table and be illuminated from below, which help with thinning and matching shades. You might have to hit it with some bleach now and again, but it does work well, for me at least.
You can take the ink off the stick with a brush if you place a drop of water on the stick first for a few moments and then thin it on the pallet or you can use the back of a piece of tile or the unglazed underside of an old stoneware dish to 'grind' a bit of pigment off for use, if you don't have dedicated stone to use.
I have both the warm and cold tone kits--and they keep plugging along after nearly 4 years. The dye remover pen is really useful as well...
What I have found is that the supposed "scale" is close for some papers, totally useless for others, and generally a guide. I has taken much experimentation of the stock of papers I use to get a feel for how they will perform. As noted, the RC papers are the trickiest--and crazy things can happen with FB papers that have been selenium toned.
FWIW from my experience, a brush or nib in combination with the pens is a better solution for them. The stuff Berg makes isn't bad--but it takes a lot of tinkering and failed ventures to get the mix just right...