Okay, first the dilution question, just to stay on topic, but it's not only unnecessary to do all the figuring, but not good practice to try to stick to a particular dilution of toner. More later; here's the formula for figuring any dilution from any other:
Q1/Q2 :: C2/C1 or… Q1 x C1 = Q2 x C2
Where
Q1 = quantity 1, Q2 = Quantity 2, C1 = Concentration 1 (%), C2 = Concentration 2 (%)
This works for anything. I use it to dilute 100-proof vodka that's been infused with lemon peel to 32% to make limoncino too
To use your example:
You have 1 liter of toner 1+9 and want an unknown volume of toner 1+20.
So: first convert your dilutions to percentages:
A 1+9 dilution has 10 total parts, one part of which is toner, so that's 1/10 or 0.1 or 10%
A 1+20 dilution has 21 total parts, one of which is toner, so that's 1/21, or approx. 0.05 or 5% (I've rounded here to a 1+19 dilution, which is way close enough)
...So replace Q1, Q2, etc. with values. C2 is your unknown.
The equation then is: 1000 x 10 = X x 5 - or - 10,000 = 5x
Divide both sides by 5 and you get: 2000 = X
So to make your liter of 1+9 dilution a 1+19, just add water to get a total volume of 2 liters.
However, you don't need to know the exact dilution of your toner to get good results. In fact, using a specific dilution of toner for a specific time will not even give you consistent results. Why? Because every print your run through the toner weakens it a bit. After a few prints, you'll need to lengthen the toning time to get the same results.
The better way is to simply make up a toning solution at the dilution you think will work best and tone a print. Keep an untoned print close by for comparison and use good lighting. Pull the print when the tone change has almost reached what you want (the print will continue toning in the rinse for a short time).
Keep track of the time. If toning times are too long, you need a stronger dilution, if too short, you need weaker. How much stronger of weaker depends on the time. Add toner concentrate (in very small amounts) or water (in larger amounts) to find a dilution that works for your paper and the amount of toning you like.
After you tone, save your toner! Filter it and store it in a bottle. It will last for years. When you're ready to tone again, filter the toner (there will be a precipitate; it's harmless) and see how it tones. If times are too slow, add more toner concentrate to replenish the solution. I've got two jugs of toner, one labeled "weak" and one labeled "strong" that have been going for 10+ years each.
This method of replenishing and reusing toner is economical and keeps you from discarding toxic selenium down the drain. However, you'll never have an exact dilution; just a dilution that gives you the degree of toning you want in a reasonable time.
Best,
Doremus