When you're working on dark hair, even though you are lifting the base with your tint, the dark hair has much more orange pigment to begin with which is exposed in the process.
If you use 12% with most colours (I think you use l'oreal?) what happens is that the peroxide will actually over-develop the dyes and they start to break down (basically bleaching the new dyes), so you end up with very brassy results.
6% gives you the most colour deposit/neutralisation, 9% gives you a little more lift but less deposit. 12% can be used with high-lift colour but will still give you very warm results on dark bases.
What I would do is use a level 7 ash e.g. 7.1+30 vol. If the client is a natural level 3, it will lift your client to a level 6, however you will expose a lot of orange pigment so I would add blue mixtone to the color, or I would instead use a stronger ash colour like inoa 7.11.
I wouldn't worry about adding extra warmth or gold to the colour, since there will already be so much warmth in the natural pigment, the result will be a warm chestnut tone.