Help with mixing dye

OK. Let's see.
6.888 is dark blonde intense red extra, right?
The 6 marks the base colour (dark blonde), the 888 is the shade (red-red-red ;))

Usually Igora colours are mixed in a 1:1 ratio, i.e. 60 ml of the dye cream and 60 ml of a developer (or more or less, depending on your hair length and thickness, and whether you just want to touch up your roots or dye the whole head.)

Choose the developer according to your needs:
3% / 10 Vol. => for "pastel dye" after bleaching or dying to a darker shade from a light natural hair colour (in this case the initial colour should be a light to medium blonde)
6% / 20 Vol. => for direct dye (tone on tone), touching up whites, lightening 0-1 shades (initial hair colour is dark blonde to light brown)
9% / 30 Vol. => lightening 1-2 shades (initial hair colour is light to medium brown)
12% / 40 Vol. => lightening 2-4 shades (initial hair colour is medium brown - black)
Try to stick with the lowest concentration to prevent damage to your hair.

40 vol is only for mixing with hilift color. Using 40 vol with a normal dye will result in more damage and much less deposit och pigments

Agreed, the 40vol will probably kill your hair and it's not good to get it on your scalp either.