Amounts/Ratio for mixing??

I color my own hair and have recently been purchasing the 'stuff' from Sallys to save some $$$$$.  I have 20 vol. creme developer (L'Oreal) and a bottle of L'Oreal Excellence toner (9BB) but I am not sure of the amounts needed for mixing.  The bottle of toner measures 2 fl. oz, but how much developer do I mix with this? Is this a 1:1 ratio, 2:1, I guess I need to know the ratio of toner vs. developer.  Can anyone help?