Hi Jackllewellyn,
If we assume the hair is a natural level 3, evan mixing a level 6 colour with 30 volume will probably only give you two levels of lift, so I wouldn't try to achieve any higher than a level 5 with a single process colour, or the results will be brassy.
As far as controlling warmth, I would add extra ash to your formula, since using 30 volume will over oxidise and 'eat up' of the artificial pigment.
If using Majirel you could use 1 tube 6,1 + 1/4 tube green or blue mix tone + 30 volume. (Use one level lighter with Majirel ash shades)
If using INOA you could use 5,17 + 30 volume.
Both of those formulas are assuming you want to take the hair to a neutral level 5.
If still want to take the hair to a level 6 you can use this method:
Lift the hair using a level 10 colour + 30 volume, applied as a virgin application. Lift the hair to a level 5 undertone and then rinse the colour.
Asses the amount of warmth being exposed, and then apply your target colour (e.g. INOA 6,1) with 20 volume, to achieve one more level of lift and maximum colour deposit.
I would only suggest this method if you are very confident with colour.
If you are in the US, L'Oreal make a high-lift brown called Majilift, but they don't sell it in to UK.
Lastly if at all possible- strand test! The best way to see how the hair will lighten and how much warmth is inside.
Adam