Jim heres how I do it... and it has worked quite well.
1000ml of distilled water (the tap water here in Austin seemed to have a negative impact on the effectiveness of the toning)
10grams of Citric acid.
then 10 DROPS (not ML) of platinum.
That would solidly tone a 7x17 Van Dyke Brown Print in about 15-20mins

then to make successive toned prints I would just add an additional 10 drops of pt to the mixture. The citric acid seemingly held its properties for usually 4 tonings of 7x17s.

When toning 4x5s, 5x7s, I could easily tone 2-3 of each in the above mixture.

when toning 8x10s I did the same thing but used 8 drops of pt for each. likely a bit much but it worked.

If I remember correctly, 10drops = 1ml roughly of solution.

Hopefully Sandy will find this and post because he was of extensive help to me in figuring out the above.

The above quantities seemed to work as well when toning in gold and palladium.
Very unique and differing results with each metal toning.