Depending on the pic we're waiting on, 9.1 could work? Or similar to the 8 level advise in colour touch my favourite toner ever was 7.1 with lowest developer.... On wet hair and watch!! Takes minutes or seconds and best applied fast or in quarters (pally watch rinse, repeat next section) you have a split second to remove before its gone from a beautiful blonde to grey. But I do mean beautiful.
Also since I trained with ct it could have all changed but from my memory ct don't lift?? I know they have 4% but that's more if you want a lighter tonal deposit than the other which will give optimum tonal deposit.
Also using cross brands doesn't give you the best finish using same brands could, so if you do this, I would advise you look at a range that tone with 3% like joico! we can tone with glaze (optimum deposit), 10 (medium deposit) and 20 (light deposit also lifts a little)
I'm all for bending the rules but breaking them is asking for trouble, panther you need to find a range that best suits you and allows you flexibility to do so, should anything go wrong on one occasion (which is all it will take) your insurance will be invalid as you haven't followed manufacturers instructions, and I hear too many times people saying well they couldn't prove it... Of course they can they would investigate and turn up unannounced to check your stock etc, should they feel thats necessary.
I'm only giving my opinion and not asking for a debate and defensive response, I don't care for it, it's just something to think about and what you do with this advise is upto you. I have been around the block, tried everything including cross branding (sorry older geeks but you don't know until you try) and I am one who learns by trying, so I've tried everything and every combination possible and realise same brands and living by theyre rules is the only way forward, go on their courses and they show you how to bend the rules and open up your colouring experiences. Good luck.